fix type hints with commonn input/output filename

Input_filename and output_filename could be None or Str. This lead
to typing issues where the static analysis type hint tool could not
check that the type colisions would not happen in runtime.

So the logic was replaced by throwing a runtime exception if either
of these valiables is ever used without first having been set.

dangerzone/common.py

@@ -14,5 +14,27 @@ class Common(object):
 
     def __init__(self) -> None:
         # Name of input and out files
-        self.input_filename: Optional[str] = None
-        self.output_filename: Optional[str] = None
+        self._input_filename: Optional[str] = None
+        self._output_filename: Optional[str] = None
+
+    @property
+    def input_filename(self) -> str:
+        if self._input_filename is None:
+            raise RuntimeError("Input filename has not been set yet.")
+        else:
+            return self._input_filename
+
+    @input_filename.setter
+    def input_filename(self, filename: str) -> None:
+        self._input_filename = filename
+
+    @property
+    def output_filename(self) -> str:
+        if self._output_filename is None:
+            raise RuntimeError("Output filename has not been set yet.")
+        else:
+            return self._output_filename
+
+    @output_filename.setter
+    def output_filename(self, filename: str) -> None:
+        self._output_filename = filename

dangerzone/gui/main_window.py

@@ -471,7 +471,7 @@ class SettingsWidget(QtWidgets.QWidget):
             self.save_lineedit.setText(os.path.basename(self.common.output_filename))
 
     def start_button_clicked(self) -> None:
-        if self.common.output_filename is None:
+        if self.save_checkbox.checkState() == QtCore.Qt.Unchecked:
             # If not saving, then save it to a temp file instead
             tmp = tempfile.mkstemp(suffix=".pdf", prefix="dangerzone_")
             self.common.output_filename = tmp[1]