Clicking "Cancel" on Media Search dialog crashes cx2 with error "VM972:1037 Uncaught TypeError: Cannot read property 'hasBeenModified' of null". As a result, the media insertion dialog closes but the background overlay remains, making the editor behind it unusable (and without a way to close other than reloading the page, closing the tab, etc.).

Steps to replicate:

1. Click on a paragraph
2. Open Media Search dialog
3. Now without doing anything click on "Cancel"

The page becomes unresponsive with an error in the console "VM972:1037 Uncaught TypeError: Cannot read property 'hasBeenModified' of null"