[Bug] Hanging 'x' function upon deletion

[makloooo]

The function 'x' will keep itself on the graph after deletion, despite being deleted from both mFunctionList (belongs to FunctionAdapter mFunctionAdapter), and functions (belongs to GraphHandler graph). Tried in the most recent branch and it's not there anymore, but I'm still a little worried.

How to reproduce error:

1. Enter 'x' as the first function in the list.
2. Enter another function as the second function in the list.
3. Delete the first function.

If you try this and it turns out it's being removed properly, please test it with differing conditions. Like, different bounds, multiple graphs, with polar graphing (would be Θ in this case), and other stuff. I just need someone to verify if it's actually gone or not.