To reproduce:
- open MITK workbench of this branch (Custom Display is default)
- open RenderWindowManager plugin
- open DICOM Editor (Custom display gets deactivated, combo box entries of the RenderWindowManager are removed)
- close Custom display without setting it's focus (just click on the X)
- open Standard Display
-> workbench crash in `mitk::Message Send`
suspicious: `listeners.assign(this->m_Listeners.begin(), this->m_Listeners.end());`: `listeners` contains one more element than `m_Listeners`