To reproduce:
- open MITK workbench of this branch (Custom Display is default)
- open RenderWindowManager plugin
- click on the Custom display to activate it explicitly
- open DICOM Editor (Custom display gets deactivated)
- open Standard Display (Standard display gets activated)
- close Custom display without setting it's focus (just click on the X)
- click on the RenderWindowManager plugin to activate it
-> workbench crash in bery::SmartPointer operator ==