To reproduce:
1. Drag out and close windows one by one
2. Try to drag out the last remaining window

Expected result: Should be possible to drag out and close the last window, leaving main window empty
Actual result: Not possible to drag out the last remaining window

Regression first introduced in: 5.5.0b8
Reproduced on: 5.5.0b8, 5.5.4f1, 5.6.1p2, 2017.1.0b9, 2017.2.0a3
Not reproduced on: 5.4.5p3, 5.5.0b7

Note: When removing all of the windows was possible, doing so prompted an ugly bug in the empty main window.