Exception raised in ColormapDialog
With recent updates of the `ColormapDialog`, I get this error from time to time:
```
Exception ignored in: <bound method ColormapDialog._itemAboutToFinalize of <silx.gui.dialog.ColormapDialog.ColormapDialog object at 0x7f12b196d438>>
Traceback (most recent call last):
  File "/users/tvincent/src/silx/build/lib.linux-x86_64-3.7/silx/gui/dialog/ColormapDialog.py", line 1201, in _itemAboutToFinalize
    self.setItem(None)
  File "/users/tvincent/src/silx/build/lib.linux-x86_64-3.7/silx/gui/dialog/ColormapDialog.py", line 1148, in setItem
    self._invalidateData()
  File "/users/tvincent/src/silx/build/lib.linux-x86_64-3.7/silx/gui/dialog/ColormapDialog.py", line 957, in _invalidateData
    if self.isVisible():
RuntimeError: wrapped C/C++ object of type ColormapDialog has been deleted
```

diff --git a/silx/gui/dialog/ColormapDialog.py b/silx/gui/dialog/ColormapDialog.py
index ead85d152..336cc47e2 100644
--- a/silx/gui/dialog/ColormapDialog.py
+++ b/silx/gui/dialog/ColormapDialog.py
@@ -1,7 +1,7 @@
 # coding: utf-8
 # /*##########################################################################
 #
-# Copyright (c) 2004-2019 European Synchrotron Radiation Facility
+# Copyright (c) 2004-2020 European Synchrotron Radiation Facility
 #
 # Permission is hereby granted, free of charge, to any person obtaining a copy
 # of this software and associated documentation files (the "Software"), to deal
@@ -82,6 +82,7 @@ import weakref
 from silx.math.combo import min_max
 from silx.gui.plot import items
 from silx.gui import icons
+from silx.gui.qt import inspect as qtinspect
 from silx.gui.widgets.ColormapNameComboBox import ColormapNameComboBox
 from silx.math.histogram import Histogramnd
 from silx.utils import deprecation
@@ -1187,17 +1188,17 @@ class ColormapDialog(qt.QDialog):
 
     def _colormapAboutToFinalize(self, weakrefColormap):
         """Callback when the data weakref is about to be finalized."""
-        if self._colormap is weakrefColormap:
+        if self._colormap is weakrefColormap and qtinspect.isValid(self):
             self.setColormap(None)
 
     def _dataAboutToFinalize(self, weakrefData):
         """Callback when the data weakref is about to be finalized."""
-        if self._data is weakrefData:
+        if self._data is weakrefData and qtinspect.isValid(self):
             self.setData(None)
 
     def _itemAboutToFinalize(self, weakref):
         """Callback when the data weakref is about to be finalized."""
-        if self._item is weakref:
+        if self._item is weakref and qtinspect.isValid(self):
             self.setItem(None)
 
     @deprecation.deprecated(reason="It is private data", since_version="0.13")
['silx/gui/dialog/ColormapDialog.py']