Fix logged NPE in ViewerUpdateMonitor constructor eclipse-platform#1500

There is a logged NPE in the constructor of ViewerUpdateMonitor when
viewerInput is null. This can happen sporadically when updating tree
elements during Viewer.setInput(null).

This commit adds a check to avoid creating children updates when the
viewer input is null.

Fixes eclipse-platform#1500

debug/org.eclipse.debug.ui/ui/org/eclipse/debug/internal/ui/viewers/model/TreeModelContentProvider.java

@@ -1185,9 +1185,10 @@ void doUpdateElement(TreePath parentPath, int modelIndex) {
 		Assert.isTrue( getViewer().getDisplay().getThread() == Thread.currentThread() );
 
 		Object parent = getElement(parentPath);
+		Object viewerInput = getViewer().getInput();
 		IElementContentProvider contentAdapter = ViewerAdapterService.getContentProvider(parent);
-		if (contentAdapter != null) {
-			ChildrenUpdate request = new ChildrenUpdate(this, getViewer().getInput(), parentPath, parent, modelIndex, contentAdapter);
+		if (viewerInput != null && contentAdapter != null) {
+			ChildrenUpdate request = new ChildrenUpdate(this, viewerInput, parentPath, parent, modelIndex, contentAdapter);
 			schedule(request);
 		}
 	}