[processing] Fix crash when adding a mesh dataset group input

Author: nirvn

src/gui/processing/qgsprocessingmeshdatasetwidget.cpp

@@ -782,14 +782,17 @@ QgsProcessingMeshDatasetGroupsParameterDefinitionWidget::QgsProcessingMeshDatase
 
   const QgsProcessingParameterMeshDatasetGroups *datasetGroupDef =
     static_cast< const QgsProcessingParameterMeshDatasetGroups *>( definition );
-  int currentIndex = mParentLayerComboBox->findData( datasetGroupDef->meshLayerParameterName() );
-  if ( currentIndex != -1 )
-    mParentLayerComboBox->setCurrentIndex( currentIndex );
-  else if ( !datasetGroupDef->meshLayerParameterName().isEmpty() )
-  {
-    // if no layer parameter candidates found, we just add the existing one as a placeholder
-    mParentLayerComboBox->addItem( datasetGroupDef->meshLayerParameterName(), datasetGroupDef->meshLayerParameterName() );
-    mParentLayerComboBox->setCurrentIndex( mParentLayerComboBox->count() - 1 );
+  if ( datasetGroupDef )
+  {
+    int currentIndex = mParentLayerComboBox->findData( datasetGroupDef->meshLayerParameterName() );
+    if ( currentIndex != -1 )
+      mParentLayerComboBox->setCurrentIndex( currentIndex );
+    else if ( !datasetGroupDef->meshLayerParameterName().isEmpty() )
+    {
+      // if no layer parameter candidates found, we just add the existing one as a placeholder
+      mParentLayerComboBox->addItem( datasetGroupDef->meshLayerParameterName(), datasetGroupDef->meshLayerParameterName() );
+      mParentLayerComboBox->setCurrentIndex( mParentLayerComboBox->count() - 1 );
+    }
   }
 
   setLayout( vlayout );