Fix crash on exit when project signal is emitted during project destr…

…uction

src/core/qgsproject.cpp

@@ -406,6 +406,8 @@ QgsProject::QgsProject( QObject *parent )
 
 QgsProject::~QgsProject()
 {
+  mIsBeingDeleted = true;
+
   clear();
   delete mBadLayerHandler;
   delete mRelationManager;
@@ -714,7 +716,12 @@ void QgsProject::clear()
   mArchive->clear();
 
   emit labelingEngineSettingsChanged();
-  emit projectColorsChanged();
+
+  if ( !mIsBeingDeleted )
+  {
+    // possibly other signals should also not be thrown on destruction -- e.g. labelEngineSettingsChanged, etc.
+    emit projectColorsChanged();
+  }
 
   // reset some default project properties
   // XXX THESE SHOULD BE MOVED TO STATUSBAR RELATED SOURCE

src/core/qgsproject.h

@@ -1502,6 +1502,8 @@ class CORE_EXPORT QgsProject : public QObject, public QgsExpressionContextGenera
 
     std::unique_ptr< QTranslator > mTranslator;
 
+    bool mIsBeingDeleted = false;
+
     friend class QgsProjectDirtyBlocker;
 
     // Required by QGIS Server for switching the current project instance

tests/src/python/test_qgsproject.py

@@ -1183,6 +1183,13 @@ def testColorScheme(self):
         self.assertEqual(len(spy), 2)
         self.assertEqual([[c[0].name(), c[1]] for c in scheme.fetchColors()], [])
 
+        # should be no signal on project destruction -- can cause a crash
+        p = QgsProject()
+        spy = QSignalSpy(p.projectColorsChanged)
+        p.deleteLater()
+        del p
+        self.assertEqual(len(spy), 0)
+
 
 if __name__ == '__main__':
     unittest.main()