Gui: [skip ci] workaround for Qt5.12.9 that causes a crash in the par…

…ameter editor for right clicks in an empty area

src/Gui/DlgParameterImp.cpp

@@ -701,10 +701,17 @@ bool ParameterValue::edit ( const QModelIndex & index, EditTrigger trigger, QEve
 void ParameterValue::contextMenuEvent ( QContextMenuEvent* event )
 {
     QTreeWidgetItem* item = currentItem();
-    if (item && item->isSelected())
+    if (item && item->isSelected()) {
         menuEdit->popup(event->globalPos());
-    else
-        menuNew->popup(event->globalPos());
+    }
+    else {
+        // There is a regression in Qt 5.12.9 where it isn't checked that a sub-menu (here menuNew)
+        // can be popped up without its parent menu (menuEdit) and thus causes a crash.
+        // A workaround is to simply call exec() instead.
+        //
+        //menuNew->popup(event->globalPos());
+        menuNew->exec(event->globalPos());
+    }
 }
 
 void ParameterValue::keyPressEvent (QKeyEvent* event)