handle None (albeit in an ugly way); also handle TypeError when converting values back to original type; update value type also in SetValue

mathentry.py

@@ -24,14 +24,20 @@ class MathEntry(wx.TextCtrl):
 
     def GetValue(self):
         val = super().GetValue()
+
+        if val == "None": #TODO?
+            return None
+
         try:
             val = self.value_type(val)
-        except ValueError:
-            pass
+        except (ValueError, TypeError) as e:
+            print(e)
+
         return val
 
 
     def SetValue(self, val):
+        self.value_type = type(val)
         val = str(val)
         super().SetValue(val)