@MrMinemeet I didn't understand this issue before but I think I understand it now. I wrote about it on this pull request last night.
Here's the excerpt:
None of the demos are working because they are either throwing a null reference exception or simply exiting immediately when the game runs.
The good news is I think I understand why it's happening. It's most likely got to do with how NuGet is resolving the package version.
See, the way a PCL bait and switch is supposed to work is that MonoGame.Extended references the PCL version of MonoGame (in this case MonoGame.Framework.Portable version 126.96.36.1998) and then the "real" game references a platform specific version of MonoGame (in this case MonoGame.Framework.DesktopGL.Core version 188.8.131.52).
The problem is (I think) when NuGet resolves which version of the DLL to use it sees that the PCL version is higher than the DesktopGL.Core version and uses the wrong one.
So to fix this we either need the DesktopGL.Core version to be the same or higher version than the PCL or to put it another way we need the PCL to be lower than the DesktopGL.Core version.
I hope that at least helps you understand why it's happening.