Support `nullable` compilation without warnings

[Remscar]

I like to use the Nullables compiler option in my projects since it helps me prevent against null dereferences way easier.

I think it would be good for this project to be fixed up so that when compiling with Nullables enabled you don't get any compiler warning.

[timmybo5]

What kind of changes does this require?

[Remscar]

Just more explicit null checks and adding ? in front of some variables.

I can knock this out in like an hour, will get to it one of these days.