Only read linter results if it exited normally
Only read linter results if it exited normally

Previously, if the check_process crashed, then checkModelError would set it to null, then checkModelFinished would run, leading to a null pointer access.

Also uses deleteLater rather than delete since these are in signals that are triggered by the process, so it may not be safe to delete.

MiniZinc/MiniZincIDE,MiniZinc/MiniZincIDE,MiniZinc/MiniZincIDE,MiniZinc/MiniZincIDE