NormIntensityWindow: fix, can be called twice

tomwer/gui/reconstruction/normalization/intensity.py

@@ -140,8 +140,9 @@ class NormIntensityWindow(qt.QMainWindow):
         self._optsWidget.setCurrentMethod(mode)
 
     def stop(self):
-        self._centralWidget.stop()
-        self._centralWidget = None
+        if self._centralWidget is not None:
+            self._centralWidget.stop()
+            self._centralWidget = None
 
     def getExtraArgs(self) -> dict:
         return self._optsWidget.getExtraInfos()