Pressing an operator before entering numbers shuts down the program.

[3rdPix]

Describe the bug
If you press an operator before writing any number, it breaks. Output error shown:

Traceback (most recent call last):
  File "D:\gui_calculator\calculator_main.py", line 114, in plus
    if ((exist_text[-1] == "+") | (exist_text[-1] == "-") | (exist_text[-1] == "*") | (exist_text[-1] == "/") | (
         ~~~~~~~~~~^^^^
IndexError: string index out of range

Expected behavior
In Windows' calculator, number 0 is already pre-written. Operator selected applies over that 0. If new operators are chosen, old operator updates while keeping the 0.

Thank you for bringing these issues to our attention. As you pointed out, there was a problem with the operands not being displayed after entering an operator, and '0' not showing up correctly when no input had been made or after a clear operation. Moreover, the error that arose when multiple operators were entered at once and the issue with the program terminating when attempting to perform a reciprocal function on zero have been acknowledged. We've modeled our corrective actions after the behavior of the Windows Calculator. We appreciate your contribution to improving our application.

To view the version of the code with the corrections applied, please refer to the latest release tag v1.2.0 and commit hash b680ede.