diff --git a/dangerzone/common.py b/dangerzone/common.py
index 8f48018..6d4d9cd 100644
--- a/dangerzone/common.py
+++ b/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
\ No newline at end of file
diff --git a/dangerzone/gui/main_window.py b/dangerzone/gui/main_window.py
index fa15f03..75ab60c 100644
--- a/dangerzone/gui/main_window.py
+++ b/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]