Unable to run from source on Windows 10

[RabJon]

First I tried the program by simply downloading the .exe and starting it. Everything worked immediately. Then I wanted to run the program from source and I followed the instructions in the README. The program also starts, but unfortunately no classes are displayed in the toolbar and no instances are displayed in the instance list. I compared the console output between .exe and source-compilation and saw that the latter throws the following TypeError:

file:///<my Path>/label_grab/resources/qml/main.qml:237: TypeError: Cannot read property 'length' of undefined. 

This happens both, when I explicitly specify a .json file with my classes, but also when I start the program without --config option, so that the default classes should be loaded.