UML Diagram disappears when dragging tab

Hello there

I've got a problem with my plugin that shows an UML diagram with UML with com.intellij.uml.

Problem is, when I have the diagram in an editor window and I drag the coresponding tab then the diagram disappears. Debugging shows, that the model (DiagramDataModel) builds up correct and even the JComponent is beeing constructed (via com.intellij.diagram.extras.DiagramExtras#createNodeComponent).

I debugged alot, but I grope in the dark. Is anybody here who has got an idea to solve that problem or even has a good area for debugging?

Greetings
Stefan