Thank you for your help! Opening the Python console made me realize I had problems with installation and that my problem was the same as in this thread: Failure to load RTSTRUCT dicom file
I have now also solved it by installing Slicer 5.2.2 instead of Slicer 5.6.2.
There was alot of red massages when I opend the console so I will not paste all of them here, but for documentation, some of the error massages I got was:
When opening slicer after installation:
[Qt] Populating font family aliases took 190 ms. Replace uses of missing font family “.AppleSystemUIFont” with one that exists to avoid this cost.
After restarting slicer after slicerRT installation:
Cannot load library /Applications/Slicer.app/Contents/Extensions-32448/SlicerRT/lib/Slicer-5.6/qt-loadable-modules/libqSlicerBeamsModule.dylib: (dlopen(/Applications/Slicer.app/Contents/Extensions-32448/SlicerRT/lib/Slicer-5.6/qt-loadable-modules/libqSlicerBeamsModule.dylib, 0x0085): Library not loaded: @rpath/libomp.dylib