Removed warning popup when canceling "add new mod"

Solves issue #1
When the file selection dialog closes without any selection it no longer causes "is no file" and "not a valid mod file" prompts to occur.

window/main_window.py

@@ -102,12 +102,16 @@ class MainWindow(QtWidgets.QMainWindow):
 
         result = dialog.getOpenFileName(filter='ZIP (*.zip)')
         self.__logger.debug(f"user selected \"{result[0]}\"")
+        if result == ('', ''):
+            self.__logger.debug("Action: \"add new mod\" was cancelled!")
+            return
         if not os.path.isfile(result[0]):
             dialog = QtWidgets.QMessageBox()
             dialog.setWindowTitle("Not a file")
             dialog.setInformativeText(f"The given file \"{result}\" did not look like a file!")
             dialog.setIcon(QtWidgets.QMessageBox.Icon.Warning)
             dialog.exec()
+            return
         if not self.parent.is_valid_mod_file(result[0]):
             dialog = QtWidgets.QMessageBox()
             dialog.setWindowTitle("Not a valid mod file")
@@ -115,6 +119,7 @@ class MainWindow(QtWidgets.QMainWindow):
                 f"The given file \"{result}\" did not look like a mod file. Is the manifest.json present?")
             dialog.setIcon(QtWidgets.QMessageBox.Icon.Warning)
             dialog.exec()
+            return
 
         self.parent.add_mod_file(result[0])