Fix NPE in ModelServiceImpl#bringToTop

ModelService#getTopLevelWindowFor is allowed to return null in some
cases but ModelServiceImpl#bringToTop does not check the return value
leading to NPE in some rare cases.

This now adds an additional check to ensure it simply breaks out in such
case.

Author: laeubi

bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/ui/internal/workbench/ModelServiceImpl.java

@@ -615,6 +615,9 @@ public void bringToTop(MUIElement element) {
 		}
 
 		MWindow window = getTopLevelWindowFor(element);
+		if (window == null) {
+			return;
+		}
 		if (window == element) {
 			if (!element.isToBeRendered()) {
 				element.setToBeRendered(true);