NPE in FencedFeedbackPanel#onRemove

There is a potential NPE in FencedFeedbackPanel.onRemove(FencedFeedbackPanel.java:155).

If a FencedFeedbackPanel instance is used two times ( i.e. wizard is replacing components on navigation) the MetaDataKey "FENCE_KEY" could be null. This causes a NPE in any further call of "onRemove".

Example Stacktrace:

Caused by: java.lang.NullPointerException
at org.apache.wicket.feedback.FencedFeedbackPanel.onRemove(FencedFeedbackPanel.java:155)
at org.apache.wicket.Component.internalOnRemove(Component.java:1102)
at org.apache.wicket.MarkupContainer.removeChildren(MarkupContainer.java:1589)
at org.apache.wicket.Component.internalOnRemove(Component.java:1110)
at org.apache.wicket.MarkupContainer.removeChildren(MarkupContainer.java:1589)
at org.apache.wicket.Component.internalOnRemove(Component.java:1110)
at org.apache.wicket.MarkupContainer.removeChildren(MarkupContainer.java:1589)
at org.apache.wicket.Component.internalOnRemove(Component.java:1110)
at org.apache.wicket.MarkupContainer.removeChildren(MarkupContainer.java:1589)
at org.apache.wicket.Component.internalOnRemove(Component.java:1110)
at org.apache.wicket.MarkupContainer.removedComponent(MarkupContainer.java:1341)
at org.apache.wicket.MarkupContainer.replace(MarkupContainer.java:739)
at org.apache.wicket.extensions.wizard.Wizard.onActiveStepChanged(Wizard.java:233)