I've just been looking at Ext 5.0 which has some changes. I'm wondering if it's compatible with GeoExt 2.0. There seems problems with GeoExt.Popup
No, GeoExt 2.0.2 (https://github.com/geoext/geoext2/releases/tag/v2.0.2, released two days ago) isn't compatible with ExtJS 5.
There is ongoing work towards a version of GeoExt2 which does suppport ExtJS 5: https://github.com/geoext/geoext2/pull/274
You may be able to use the intermediate code in that branch to see if it helps with the popup, but again, it isn't officially released yet.