Caused by: java.lang.NoSuchMethodError: java.util.Set.java_util_Collection_contains(Ljava/lang/Object;)Z

I was changing another part of the application when the something
suddenly broke.

Caused by: java.lang.NoSuchMethodError:
java.util.Set.java_util_Collection_contains(Ljava/lang/Object;)Z
at
com.google.gwt.user.client.ui.RootPanel.isInDetachList(RootPanel.java:
231)
at com.google.gwt.user.client.ui.Widget.removeFromParent(Widget.java:
152)
at
com.google.gwt.user.client.ui.DockLayoutPanel.insert(DockLayoutPanel.java:
358)
at
com.google.gwt.user.client.ui.DockLayoutPanel.addNorth(DockLayoutPanel.java:
168)
at
com.kk_electronic.kkpilot.core.ui.WebPageLayout_UIBinderImpl.createAndBindUi(WebPageLayout_UIBinderImpl.java:
21)
...

Has anybody experienced anything similar?

The change I made was making a JavaScriptObject that implemented
List<E>. can this somehow has caused the problem?

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