GafferUITest : Fix assertNodeUIsHaveExpectedLifetime() test

This test was raising an exception for nodes that are supposed to be
"invisible", and therefore had no `NodeGadget`.

Changes.md

@@ -10,6 +10,7 @@ Fixes
 -----
 
 - Render, InteractiveRender : Added default node name arguments to the compatibility shims for removed subclasses such as ArnoldRender.
+- GafferUITest : Fixed `assertNodeUIsHaveExpectedLifetime()` test for invisible nodes.
 
 1.5.0.1 (relative to 1.5.0.0)
 =======

python/GafferUITest/TestCase.py

@@ -169,11 +169,13 @@ def assertNodeUIsHaveExpectedLifetime( self, module ) :
 			weakScript = weakref.ref( script )
 
 			nodeGadget = GafferUI.NodeGadget.create( script["node"] )
-			weakNodeGadget = weakref.ref( nodeGadget )
+			if nodeGadget :
+				weakNodeGadget = weakref.ref( nodeGadget )
+				del nodeGadget
+				self.assertIsNone( weakNodeGadget() )
 
-			del window, nodeUI, nodeGadget
+			del window, nodeUI
 			self.assertIsNone( weakNodeUI() )
-			self.assertIsNone( weakNodeGadget() )
 
 			del script
 			self.assertIsNone( weakScript() )