Cache default value in editorwidget

Makes sure that repeated calls return the same value for a given editor widget.
This avoids unstable behavior and unrequired updates of serial numbers.

src/gui/editorwidgets/core/qgseditorwidgetwrapper.cpp

@@ -42,7 +42,9 @@ QgsField QgsEditorWidgetWrapper::field() const
 
 QVariant QgsEditorWidgetWrapper::defaultValue() const
 {
-  return layer()->dataProvider()->defaultValue( mFieldIdx );
+  mDefaultValue = layer()->dataProvider()->defaultValue( mFieldIdx );
+
+  return mDefaultValue;
 }
 
 QgsEditorWidgetWrapper* QgsEditorWidgetWrapper::fromWidget( QWidget* widget )

src/gui/editorwidgets/core/qgseditorwidgetwrapper.h

@@ -224,14 +224,15 @@ class GUI_EXPORT QgsEditorWidgetWrapper : public QgsWidgetWrapper
      */
     virtual void updateConstraintWidgetStatus();
 
+  private:
     /**
      * Boolean storing the current validity of the constraint for this widget.
      */
     bool mValidConstraint;
 
-  private:
     int mFieldIdx;
     QgsFeature mFeature;
+    mutable QVariant mDefaultValue; // Cache default value, we don't want to retrieve different serial numbers if called repeatedly
 };
 
 // We'll use this class inside a QVariant in the widgets properties