Fixed Time and Date pickers, double setting does not occur

Change-Id: I9081c156e616ae0cb47f55a65917555da4958000
Flag switch to invalid screen during development.

This fixes Trac #530. During development you will now get an error
dialog in the blocks editor when you attempt to open a screen that
doesn’t exist.

Change-Id: I23f649f6db77a63e809ddb01325db6feb0ab84fb