Channel locking and discard: Choosing "No" or closing dialog makes template composer hang

To reproduce:

• Go to the channel manager and edit a channel
• Click one of the buttons: "Discard all changes" or "Unlock channel"
• In the dialog that shows up click "No" or the [x] to close the dialog (or press Escape)

Expected result: Return to the state before clicking "Discard all changes" or "Unlock channel"
Actual result: All buttons get disabled, and the loading indicator is displayed in the preview window.