Nested modal windows introduce broken key-event behavior

Reproduction path on Firefox:

• Open Image Upload dialog (notice that the contextmenu of the tree is still open in the background)
• Press enter (this call will actually be executed on the contextmenu, not the dialog)
• Press escape (the dialog will close and a new dialog is opened straight away)
• Press escape again (Dialog is closed but a js error in thrown

From now on the application will always throw a javascript error when escape is pressed for open dialogs.