Move preference dialog to separate file

Closes #335 (closed)

b_asic/scheduler_gui/main_window.py

@@ -979,7 +979,8 @@ class ScheduleMainWindow(QMainWindow, Ui_MainWindow):
         """Save preferences of Latency color per type in settings"""
         settings = QSettings()
         for key, color in self._color_per_type.items():
-            self._graph._color_change(color, key)
+            if self._graph:
+                self._graph._color_change(color, key)
             self._converted_color_per_type[key] = color.name()
         settings.setValue(
             f"scheduler/preferences/{LATENCY_COLOR_TYPE.name}",