Incorrect behavior on "Cancel" in save prompt

[per1234]

Using Arduino IDE 1.8.9 Hourly Build 2019-01-24 07:33 and Arduino IDE 1.9.0-beta build 101 with Windows 10 64 bit

1. Open a sketch
2. Modify the sketch
3. Close the window by clicking the "X" button at the top right corner of the window. The issue does not occur if this is done via File > Quit. This must be the last IDE window open.
4. From the save prompt, click "Cancel". The expected behavior is that the the sketch will remain open and will not be saved. What happens instead is the sketch is closed without saving.

The issue does not occur with Arduino IDE 1.8.8.

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Likely related to: #8413