When I open a page in the visual editor, it almost always throws this error:

org.eclipse.jface.util.Assert$AssertionFailedException: Assertion failed:
at org.eclipse.jface.util.Assert.isTrue(Assert.java:185)
at org.eclipse.jface.util.Assert.isTrue(Assert.java:170)
at org.jboss.tools.jst.jsp.jspeditor.JSPMultiPageEditorPart.setActivePage(JSPMultiPageEditorPart.java:276)
at org.jboss.tools.jst.jsp.jspeditor.JSPMultiPageEditor.createPages(JSPMultiPageEditor.java:452)
at org.jboss.tools.jst.jsp.jspeditor.JSPMultiPageEditorPart.createPartControl(JSPMultiPageEditorPart.java:125)
at org.eclipse.ui.internal.EditorReference.createPartHelper(EditorReference.java:661)
at org.eclipse.ui.internal.EditorReference.createPart(EditorReference.java:428)
at org.eclipse.ui.internal.WorkbenchPartReference.getPart(WorkbenchPartReference.java:594)

If I close the window and re-open the file a second time, it succeeds.

I'm running the suggested version of WTP (with the correct dependencies, AFAIK).