Fix crash when switching back to Koch Snowflake plugin

Options widget was being recreated. When the previous widget is deleted, it deletes all of its children, including self.iterationsSlider.

Maybe make sure getOptionsWidget is only called once and cache its result in GeneratePlugin

src/plugins/koch.py

@@ -126,9 +126,13 @@ if __name__ == '__main__':
 
 class KochSnowflakePlugin(LSystemPlugin):
     displayName = "Koch Snowflake"
-
+    _optionsWidget = None
+    
     def getOptionsWidget(self):
-        widget = QtGui.QWidget()
+        if self._optionsWidget:
+            return self._optionsWidget
+
+        widget = self._optionsWidget = QtGui.QWidget()
 
         self.blockTypeButton = BlockTypeButton()
         self.blockTypeButton.editorSession = self.editorSession