Silly, Mapp is definitely a speilling error

src/app/qgsgeometryvalidationdock.cpp

@@ -34,7 +34,7 @@ email                : matthias@opengis.ch
 QgsGeometryValidationDock::QgsGeometryValidationDock( const QString &title, QgsMapCanvas *mapCanvas, QgisApp *parent, Qt::WindowFlags flags )
   : QgsDockWidget( title, parent, flags )
   , mMapCanvas( mapCanvas )
-  , mApp( parent )
+  , mQgisApp( parent )
 {
   setupUi( this );
 
@@ -153,7 +153,7 @@ void QgsGeometryValidationDock::onRowsInserted()
 {
   if ( !isVisible() )
   {
-    mApp->addDockWidget( Qt::RightDockWidgetArea, this );
+    mQgisApp->addDockWidget( Qt::RightDockWidgetArea, this );
   }
   raise();
 }

src/app/qgsgeometryvalidationdock.h

@@ -72,7 +72,7 @@ class QgsGeometryValidationDock : public QgsDockWidget, public Ui_QgsGeometryVal
     QgsGeometryValidationService *mGeometryValidationService = nullptr;
     QButtonGroup *mZoomToButtonGroup = nullptr;
     QgsMapCanvas *mMapCanvas = nullptr;
-    QgisApp *mApp = nullptr;
+    QgisApp *mQgisApp = nullptr;
     QgsCoordinateTransform mLayerTransform;
     QModelIndex currentIndex() const;
     QgsRubberBand *mFeatureRubberband = nullptr;