Did you set a breakpoint to see if there's anything obviously null?
It's a long shot, but when I first started with MG I saw an error just like that when I forgot to set the MonoGame.Framework.Portable reference to have
<PrivateAssets>all</PrivateAssets> in my shared library. Without it, the Portable binaries were being copied to the output instead of the platform binaries, so
Content (among most other things) was null since there was no implementation.