Program crashes if Sudoku is unsolvable

[TimonGaertner]

If you click on solve and the Sudoku is unsolvable the program crashes.

What happens if you click on solve:

1. start method of solve.py gets called (no issue)
2. start method calls solve method and gives the current board to it (no issue)
3. solve method tries to solve (issue)
4. If solve method can't solve the sudoku it returns False (should be ok)

ends here, it should be:

5. the start method closes and the solved sudoku should open in a new window (with the solutionWindow() in UI.py-> where the inputted sudoku should be with "0" fields (the fields that weren't inputted) -> but 5th step doesn't start