popOverShell may be null when PopOverShell "hide" is called

[jentfoo]

This came in from a customer issue today. The original stack trace is this: SEVERE Could not execute effect! com.readytalk.swt.effects.FadeEffect null com.readytalk.swt.widgets.notifications.PopOverShell.hide (line 461) com.readytalk.swt.widgets.notifications.PopOverShelll.fadeComplete (line 474) com.readytalk.swt.effects.FadeEffect.effectComplete (line 94) com.readytalk.swt.effects.LinkableEffect.completeEffect (line 81) com.readytalk.swt.effects.LinkableEffect.run (line 154) org.eclipse.swt.widgets.Display.bC (native) org.eclipse.swt.widgets.Display.messageProc (native) org.eclipse.swt.internal.win32.OS.DispatchMessageW org.eclipse.swt.internal.win32.OS.a (native) org.eclipse.swt.widgets.Display.adi (native)