When taking the following steps I get the error:
Runtime Error 91
Object variable or With block variable not set

I open QDK
Create a new game
Select Rooms in the tree on the left
Click the edit button on the right without clicking anywhere else.

While it may seem like a silly bug, it still should be fixed in case someone does come across it while actually creating a game.

I also just found out that it happens after creating a room, selecting the objects tab and hitting the same edit button in that section.

Thanks for this, I have added it to the list of bugs to fix.