Avoid NPE from find/replace overlay when closing workbench

A missing null check when performing an asynchronous execution on the
display of the find/replace overlay's shell leads to a
NullPointerException at least when closing a workbench. This change adds
an according null check.

bundles/org.eclipse.ui.workbench.texteditor/src/org/eclipse/ui/internal/findandreplace/overlay/FindReplaceOverlay.java

@@ -221,12 +221,16 @@ private void toggleToolItem(ToolItem toolItem) {
 	private ControlListener shellMovementListener = new ControlListener() {
 		@Override
 		public void controlMoved(ControlEvent e) {
-			getShell().getDisplay().asyncExec(() -> positionToPart());
+			if (getShell() != null) {
+				getShell().getDisplay().asyncExec(() -> positionToPart());
+			}
 		}
 
 		@Override
 		public void controlResized(ControlEvent e) {
-			getShell().getDisplay().asyncExec(() -> positionToPart());
+			if (getShell() != null) {
+				getShell().getDisplay().asyncExec(() -> positionToPart());
+			}
 		}
 	};