add logging to fit exception to retain more info and find problem with #56; all references to current graph are either None or id, should close #57

src/gui_qt/fit/fitwindow.py

@@ -49,7 +49,7 @@ class QFitDialog(QtWidgets.QWidget, Ui_FitDialog):
         self._func_list = {}
         self._complex = {}
 
-        self.connected_figure = ''
+        self.connected_figure = None
 
         self.model_frame.hide()
         self.preview_button.hide()