QmlDesigner: Fix changePreviewImageSize

If the size is invalid set the default size.
Invalid size most likely means the setting was removed.

Change-Id: I3f1dad5bdac799673d37036099d954caab9aca19
Reviewed-by: Michael Winkelmann <michael.winkelmann@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

share/qtcreator/qml/qmlpuppet/qml2puppet/instances/qt5previewnodeinstanceserver.cpp

@@ -122,6 +122,9 @@ void Qt5PreviewNodeInstanceServer::changePreviewImageSize(
 {
     m_previewSize = command.size;
 
+    if (!command.size.isValid())
+        m_previewSize = {160, 160};
+
     collectItemChangesAndSendChangeCommands();
 }