fixup! ContextTracker : Add asynchronous update mechanism

Moved the check for a valid ScriptNode to `updateInBackground()`, since it may have been destroyed while we waited for the idle event.

python/GafferUITest/ContextTrackerTest.py

@@ -711,6 +711,7 @@ def testAcquireForFocus( self ) :
 		self.assertTrue( tracker.isActive( script["add2" ] ) )
 
 		script.setFocus( None )
+		self.waitForIdle()
 		self.assertFalse( tracker.isActive( script["add1" ] ) )
 		self.assertFalse( tracker.isActive( script["add2" ] ) )
 

src/GafferUI/ContextTracker.cpp

@@ -214,17 +214,6 @@ void ContextTracker::scheduleUpdate()
 	// Cancel old update.
 	m_updateTask.reset();
 
-	if( !m_node || !m_node->scriptNode() )
-	{
-		// Don't need or can't use a BackgroundTask (the latter case being when
-		// a ScriptNode is being destroyed). Just do the update directly on the
-		// UI thread.
-		m_nodeContexts.clear();
-		m_plugContexts.clear();
-		changedSignal()( *this );
-		return;
-	}
-
 	// Arrange to do the update on the next idle event. This allows us to avoid
 	// redundant restarts when `plugDirtied()` or `contextChanged()` is called
 	// multiple times in quick succession.
@@ -255,10 +244,21 @@ void ContextTracker::updateInBackground()
 		}
 	}
 
+	if( toVisit.empty() || !m_node->scriptNode() )
+	{
+		// Don't need or can't use a BackgroundTask (the latter case being when
+		// a ScriptNode is being destroyed). Just do the update directly on the
+		// UI thread.
+		m_nodeContexts.clear();
+		m_plugContexts.clear();
+		changedSignal()( *this );
+		return;
+	}
+
 	Context::Scope scopedContext( contextCopy.get() );
 	m_updateTask = ParallelAlgo::callOnBackgroundThread(
 
-		/* subject = */ toVisit.empty() ? nullptr : toVisit.back().first,
+		/* subject = */ toVisit.back().first,
 
 		// OK to capture `this` without incrementing reference count, because
 		// ~UpstreamContext cancels background task and waits for it to