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