silx.gui.plot.tools.profile: Fixed profile error when plot item is None

[t20100]

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ProfileRoiMixIn._setPlotItem did not support None, which can lead to errors:

ERROR:silx.app.view.main:<class 'TypeError'> cannot create weak reference to 'NoneType' object   File "silx/gui/plot/tools/profile/manager.py", line 772, in __activeImageChanged
    self.setPlotItem(item)
  File "silx/gui/plot/tools/profile/manager.py", line 942, in setPlotItem
    self.requestUpdateAllProfile()
  File "silx/gui/plot/tools/profile/manager.py", line 844, in requestUpdateAllProfile
    self.requestUpdateProfile(roi)
  File "silx/gui/plot/tools/profile/manager.py", line 870, in requestUpdateProfile
    profileRoi._setPlotItem(None)
  File "silx/gui/plot/tools/profile/core.py", line 129, in _setPlotItem
    self.__plotItem = weakref.ref(plotItem)
                      ^^^^^^^^^^^^^^^^^^^^^