Don't set cleared data defined properties to NULL in expression editor

Fixes #18638

Author: nyalldawson

src/gui/qgspropertyoverridebutton.cpp

@@ -591,7 +591,11 @@ void QgsPropertyOverrideButton::showExpressionDialog()
 {
   QgsExpressionContext context = mExpressionContextGenerator ? mExpressionContextGenerator->createExpressionContext() : QgsExpressionContext();
 
-  QgsExpressionBuilderDialog d( const_cast<QgsVectorLayer *>( mVectorLayer ), mProperty.asExpression(), this, QStringLiteral( "generic" ), context );
+  // build sensible initial expression text - see https://issues.qgis.org/issues/18638
+  QString currentExpression = ( mProperty.propertyType() == QgsProperty::StaticProperty && !mProperty.staticValue().isValid() ) ? QString()
+                              : mProperty.asExpression();
+
+  QgsExpressionBuilderDialog d( const_cast<QgsVectorLayer *>( mVectorLayer ), currentExpression, this, QStringLiteral( "generic" ), context );
   if ( d.exec() == QDialog::Accepted )
   {
     mExpressionString = d.expressionText().trimmed();