Alright after some digging around and compiling Monogame myself I found out that the problem was me being stupid. The reason I was getting those crashes was because the Graphics Device was null, because I never called Run on my game instance when running the editor. I did this because Monogame.Forms already handles window creation and graphics device initialization and so I just called the Initialize, Update and Draw methods of my class inside the Monogame.Forms methods.
Every other resource worked correctly because when initializing the editor I pass the Monogame.Forms object’s GraphicsDevice to my class, and use that when drawing sprites to the screen. And since I am using FromStream to load textures I never needed the GraphicsDevice. But as I’m using the Content Pipeline to load the spritefonts it was having the error I reported above in the editor, but not in the game (as that initialized the GraphicsDevice when Run was called).
So what I ended up doing was passing the Monogame.Forms’s UpdateWindow object’s services to where I initialize my ContentManagers, and instantiating them through the constructor that takes an IServiceProvider object. This solved my issue.
Thank you all for you help. 