[GEE Time Series Explorer app] fix custom collections

[Andreas Rabe]

E.g. click on Landsat Combined:

‌

Traceback (most recent call last):
File "D:\source\QGISPlugIns\enmap-box\enmapbox\coreapps\geetimeseriesexplorerapp\geetimeseriesexplorerdockwidget.py", line 540, in onCollectionClicked