Basic patch for Realms screen NPE

[ah-OOG-ah]

Opening the Realms screen, clicking "Buy Realm", and then hitting Esc to exit and entering a world will eventually cause an NPE+crash when the HTTP request returns and it tries to display a realms GUI. This adds a simple null check to avert that. It still kicks you out of the world, but you can just log back in instead of restarting the client.

Technically the proper fix would be to stop whatever Realms screen is trying to appear from showing in the first place, but that's much harder and would still leave an NPE for someone else to footgun on.

Tested in full pack and it launched, although the actual functionality couldn't be tested as GTNH removes the realms button.

[ah-OOG-ah]

After fixing the copy-paste error, it seems to only occasionally fix the issue as drawBackground(int) also uses this.mc. I don't think this.mc should ever change; methods that set it should set it to the same thing on every GuiScreen. I would guess that some/all GuiScreens briefly have this.mc uninitialized, so when rendering expects a this.mc the moment the screen changes it NPE's. The patch as written reduces this chance but doesn't eliminate it.

[ah-OOG-ah]

Gonna be a bit busy so I can't complete this in the forseeable future. If someone else wants to pick this into a new PR the code technically works, just isn't as comprehensive a fix as I'd like