diff --git a/python/core/auto_generated/qgsarchive.sip.in b/python/core/auto_generated/qgsarchive.sip.in
index fe9930043a9f..1877d1babc10 100644
--- a/python/core/auto_generated/qgsarchive.sip.in
+++ b/python/core/auto_generated/qgsarchive.sip.in
@@ -9,6 +9,7 @@
 
 
 
+
 class QgsArchive
 {
 %Docstring
diff --git a/python/core/auto_generated/qgsnetworkcontentfetcherregistry.sip.in b/python/core/auto_generated/qgsnetworkcontentfetcherregistry.sip.in
index 62fb76f650a9..f51504fb5657 100644
--- a/python/core/auto_generated/qgsnetworkcontentfetcherregistry.sip.in
+++ b/python/core/auto_generated/qgsnetworkcontentfetcherregistry.sip.in
@@ -11,8 +11,6 @@
 
 
 
-
-
 class QgsFetchedContent : QObject
 {
 %Docstring
diff --git a/src/core/qgsapplication.cpp b/src/core/qgsapplication.cpp
index 72ad34a36b4d..3ea27fe61eb1 100644
--- a/src/core/qgsapplication.cpp
+++ b/src/core/qgsapplication.cpp
@@ -130,6 +130,7 @@ const char *QgsApplication::QGIS_ORGANIZATION_DOMAIN = "qgis.org";
 const char *QgsApplication::QGIS_APPLICATION_NAME = "QGIS3";
 
 QgsApplication::ApplicationMembers *QgsApplication::sApplicationMembers = nullptr;
+QgsAuthManager *QgsApplication::sAuthManager = nullptr;
 
 QgsApplication::QgsApplication( int &argc, char **argv, bool GUIenabled, const QString &profileFolder, const QString &platformName )
   : QApplication( argc, argv, GUIenabled )
@@ -1167,7 +1168,6 @@ QgsAuthManager *QgsApplication::authManager()
   else
   {
     // no QgsApplication instance
-    static QgsAuthManager *sAuthManager = nullptr;
     if ( !sAuthManager )
       sAuthManager = QgsAuthManager::instance();
     return sAuthManager;
@@ -1177,7 +1177,11 @@ QgsAuthManager *QgsApplication::authManager()
 
 void QgsApplication::exitQgis()
 {
-  delete QgsApplication::authManager();
+  // don't create to delete
+  if ( instance() )
+    delete instance()->mAuthManager;
+  else
+    delete sAuthManager;
 
   //Ensure that all remaining deleteLater QObjects are actually deleted before we exit.
   //This isn't strictly necessary (since we're exiting anyway) but doing so prevents a lot of
diff --git a/src/core/qgsapplication.h b/src/core/qgsapplication.h
index d267c02b7a28..be9708908410 100644
--- a/src/core/qgsapplication.h
+++ b/src/core/qgsapplication.h
@@ -885,6 +885,8 @@ class CORE_EXPORT QgsApplication : public QApplication
     // ... but in case QgsApplication is never instantiated (eg with custom designer widgets), we fall back to static members
     static ApplicationMembers *sApplicationMembers;
 
+    static QgsAuthManager *sAuthManager;
+
     static ApplicationMembers *members();
 };
 
diff --git a/src/core/qgsarchive.cpp b/src/core/qgsarchive.cpp
index da1f53e16958..2fac26d807fe 100644
--- a/src/core/qgsarchive.cpp
+++ b/src/core/qgsarchive.cpp
@@ -20,7 +20,6 @@
 #include "qgsziputils.h"
 #include "qgsmessagelog.h"
 #include "qgsauxiliarystorage.h"
-#include <iostream>
 
 #include <QStandardPaths>
 #include <QUuid>
@@ -32,7 +31,7 @@ QgsArchive::QgsArchive()
 
 QgsArchive::QgsArchive( const QgsArchive &other )
   : mFiles( other.mFiles )
-  ,  mDir( new QTemporaryDir() )
+  , mDir( new QTemporaryDir() )
 {
 }
 
diff --git a/src/core/qgsarchive.h b/src/core/qgsarchive.h
index 7fb0f1c54db3..f7b40204f260 100644
--- a/src/core/qgsarchive.h
+++ b/src/core/qgsarchive.h
@@ -20,6 +20,7 @@
 #define QGSARCHIVE_H
 
 #include "qgis_core.h"
+
 #include <QStringList>
 #include <QTemporaryFile>
 #include <QTemporaryDir>
diff --git a/src/core/qgscoordinatereferencesystem.cpp b/src/core/qgscoordinatereferencesystem.cpp
index c26e9cadd36b..82281690dbcd 100644
--- a/src/core/qgscoordinatereferencesystem.cpp
+++ b/src/core/qgscoordinatereferencesystem.cpp
@@ -21,7 +21,6 @@
 #include <cmath>
 
 #include <QDir>
-#include <QTemporaryFile>
 #include <QDomNode>
 #include <QDomElement>
 #include <QFileInfo>
diff --git a/src/core/qgsnetworkcontentfetcherregistry.h b/src/core/qgsnetworkcontentfetcherregistry.h
index c44b1d1e24e5..e96a005c65f3 100644
--- a/src/core/qgsnetworkcontentfetcherregistry.h
+++ b/src/core/qgsnetworkcontentfetcherregistry.h
@@ -23,15 +23,12 @@
 #include <QMap>
 #include <QMutex>
 #include <QNetworkReply>
-#include <QTemporaryFile>
 
 #include "qgis_core.h"
-
-class QTemporaryFile;
-
 #include "qgstaskmanager.h"
 #include "qgsnetworkcontentfetchertask.h"
 
+class QTemporaryFile;
 
 /**
  * \class QgsFetchedContent
diff --git a/src/core/qgsproject.h b/src/core/qgsproject.h
index 9a51ce645875..2dcbbe6b66e6 100644
--- a/src/core/qgsproject.h
+++ b/src/core/qgsproject.h
@@ -1507,6 +1507,9 @@ class CORE_EXPORT QgsProject : public QObject, public QgsExpressionContextGenera
 
     friend class QgsProjectDirtyBlocker;
 
+    // Required to avoid creating a new project in it's destructor
+    friend class QgsProviderRegistry;
+
     // Required by QGIS Server for switching the current project instance
     friend class QgsConfigCache;
 };
diff --git a/src/core/qgsproviderregistry.cpp b/src/core/qgsproviderregistry.cpp
index b750d5d0bed0..c32e43509745 100644
--- a/src/core/qgsproviderregistry.cpp
+++ b/src/core/qgsproviderregistry.cpp
@@ -264,7 +264,9 @@ typedef void cleanupProviderFunction_t();
 
 void QgsProviderRegistry::clean()
 {
-  QgsProject::instance()->removeAllMapLayers();
+  // avoid recreating a new project just to clean it
+  if ( QgsProject::sProject )
+    QgsProject::instance()->removeAllMapLayers();
 
   Providers::const_iterator it = mProviders.begin();
 
diff --git a/src/core/qgssettings.cpp b/src/core/qgssettings.cpp
index ad2249471346..3feee35e14f0 100644
--- a/src/core/qgssettings.cpp
+++ b/src/core/qgssettings.cpp
@@ -15,6 +15,7 @@
 
 
 #include <cstdlib>
+
 #include <QFileInfo>
 #include <QSettings>
 #include <QDir>
diff --git a/src/crssync/CMakeLists.txt b/src/crssync/CMakeLists.txt
index 8c31ad768551..a82b22603b68 100644
--- a/src/crssync/CMakeLists.txt
+++ b/src/crssync/CMakeLists.txt
@@ -2,6 +2,7 @@ ADD_EXECUTABLE(crssync main.cpp)
 
 INCLUDE_DIRECTORIES(
   ../core
+  ../core/expression
   ../core/geometry
   ../core/metadata
   ${CMAKE_BINARY_DIR}/src/core
diff --git a/src/crssync/main.cpp b/src/crssync/main.cpp
index 99de960bef25..9b8270d22497 100644
--- a/src/crssync/main.cpp
+++ b/src/crssync/main.cpp
@@ -18,6 +18,7 @@
 #include "qgsapplication.h"
 #include "qgscoordinatereferencesystem.h"
 #include "qgsconfig.h"
+#include "qgsproject.h"
 
 #include <QRegExp>
 
@@ -84,5 +85,7 @@ int main( int argc, char **argv )
     std::cout << -res << " CRSs could not be updated." << std::endl;
   }
 
-  exit( 0 );
+  QgsApplication::exitQgis();
+
+  return 0;
 }
diff --git a/src/gui/qgscolorbutton.cpp b/src/gui/qgscolorbutton.cpp
index 9a9ec06dd298..4e55a340ba46 100644
--- a/src/gui/qgscolorbutton.cpp
+++ b/src/gui/qgscolorbutton.cpp
@@ -26,7 +26,6 @@
 #include "qgsguiutils.h"
 
 #include <QPainter>
-#include <QTemporaryFile>
 #include <QMouseEvent>
 #include <QMenu>
 #include <QClipboard>
diff --git a/src/gui/qgscolorbutton.h b/src/gui/qgscolorbutton.h
index b6ac1b2331ba..1a3c43d48ae8 100644
--- a/src/gui/qgscolorbutton.h
+++ b/src/gui/qgscolorbutton.h
@@ -17,7 +17,6 @@
 
 #include <QColorDialog>
 #include <QToolButton>
-#include <QTemporaryFile>
 #include "qgis_gui.h"
 #include "qgis_sip.h"
 
diff --git a/src/gui/symbology/qgsstyleexportimportdialog.cpp b/src/gui/symbology/qgsstyleexportimportdialog.cpp
index 985548fb6aee..f7de5985c955 100644
--- a/src/gui/symbology/qgsstyleexportimportdialog.cpp
+++ b/src/gui/symbology/qgsstyleexportimportdialog.cpp
@@ -33,8 +33,11 @@
 #include <QCloseEvent>
 #include <QFileDialog>
 #include <QMessageBox>
+#include <QProgressDialog>
 #include <QPushButton>
 #include <QStandardItemModel>
+#include <QNetworkAccessManager>
+#include <QNetworkReply>
 
 
 QgsStyleExportImportDialog::QgsStyleExportImportDialog( QgsStyle *style, QWidget *parent, Mode mode )
diff --git a/src/gui/symbology/qgsstyleexportimportdialog.h b/src/gui/symbology/qgsstyleexportimportdialog.h
index db0617d2065d..e1cfcd41a625 100644
--- a/src/gui/symbology/qgsstyleexportimportdialog.h
+++ b/src/gui/symbology/qgsstyleexportimportdialog.h
@@ -19,11 +19,6 @@
 
 #include <QDialog>
 #include <QUrl>
-#include <QProgressDialog>
-#include <QTemporaryFile>
-#include <QNetworkAccessManager>
-#include <QNetworkReply>
-#include <QStandardItem>
 
 #include "ui_qgsstyleexportimportdialogbase.h"
 #include "qgis_gui.h"
@@ -36,6 +31,10 @@ class QgsStyle;
 class QgsStyleGroupSelectionDialog;
 class QgsTemporaryCursorOverride;
 class QgsStyleModel;
+class QTemporaryFile;
+class QProgressDialog;
+class QNetworkAccessManager;
+class QNetworkReply;
 
 /**
  * \ingroup gui