Random deferred binding error at starup in dev mode

I get the following error at dev mode startup often.  It seems random when it occurs but is a productivity killer...some times it will happen several times in a row...then not happen for a while.  Any ideas as to the cause?

SEVERE: Deferred binding failed for 'com.sencha.gxt.widget.core.client.tree.Tree$TreeAppearance' (did you forget to inherit a required module?)

java.lang.RuntimeException: Deferred binding failed for 'com.sencha.gxt.widget.core.client.tree.Tree$TreeAppearance' (did you forget to inherit a required module?): at

com.google.gwt.dev.shell.GWTBridgeImpl.create(GWTBridgeImpl.java:53)

com.google.gwt.core.shared.GWT.create(GWT.java:72)

com.google.gwt.core.client.GWT.create(GWT.java:86)

com.sencha.gxt.widget.core.client.tree.Tree.<init>(Tree.java:571)

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