Flameshot: Windows - invalid date formats cause the software to crash

[TuxGamer]

Flameshot version
v0.6.0-26-g94daa4f with Qt 5.9.7
Downloaded from here: https://transfer.sh/Y81UM/flameshot_0.6.0_win_X64.zip

Describe the bug
Flameshot crashes ("has stopped working") when entering an invalid date format, such as a percent sign without a date specifier after it. Entering other strings like "Screenshot_" beforehand is fine.

See this gif: https://i.imgur.com/T6a3HHm.gifv

To Reproduce
Open up the configuration window and select the Filename Editor tab. Paste the following text in the text box at the bottom: "Screenshot_%"

Expected behavior
The application does not crash and silently ignores the error. Ideally, it performs a validation when clicking the Save button at the bottom and opens up an alert window, if the format is invalid.

System Information
Windows 10 Enterprise, Version 1709, build 16299.785,
Windows 8.1 Pro, Build 9600,
Windows Server 2016 Standard Edition, Version 1607, Build 14393.2125

[thepurple]

Is fixed in our fork, will be merged a bit later.