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)