Fix failing StackRendererTest Test

GC of the event must not be null.

Fixes: eclipse-platform/eclipse.platform.swt#1515

Author: BeckerWdf

tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/workbench/renderers/swt/StackRendererTest.java

@@ -52,6 +52,7 @@
 import org.eclipse.swt.SWT;
 import org.eclipse.swt.custom.CTabFolder;
 import org.eclipse.swt.custom.CTabItem;
+import org.eclipse.swt.graphics.GC;
 import org.eclipse.swt.graphics.Image;
 import org.eclipse.swt.graphics.Rectangle;
 import org.eclipse.swt.widgets.Composite;
@@ -540,7 +541,9 @@ public void testOnboardingIsHiddenWhenEditorOpened() {
 		partStack.getChildren().add(part1);
 		partStack.setSelectedElement(part1);
 
-		tabFolder.notifyListeners(SWT.Paint, new Event());
+		Event event = new Event();
+		event.gc = new GC(tabFolder);
+		tabFolder.notifyListeners(SWT.Paint, event);
 
 		expected = new Rectangle(StackRenderer.ONBOARDING_SPACING, StackRenderer.ONBOARDING_TOP_SPACING, 0, 0);
 		assertEquals(expected, outerOnboardingComposite.getBounds());