setPage method calls getStream and this method tries to check whether URL is redirected (among the other thing like setting language and content type). This can be time consuming (in the case of slow connection or it can wait until timeout expires for unreachable URL) and sometimes even may cause deadlocks. For example if connection requires authentication you may want to display prompt for input data but AWT will be blocked. Another example is when the HTTP connection is provided by the same process and attempts to use GUI.
In NetBeans this problem is workarounded by calling setPage from non AWT thread. But to force reload on the same URL we also use recommended way how to force reload by calling doc.putProperty(Document.StreamDescriptionProperty, null); but as JEditorPane.run accesses this property in AWT thread (line 656 in JEditorPane.java JDK6u2) without null check it can result in NPE - it is typical race condition. Is there any workaround you could recommend us?

New race condition issue is http://www.netbeans.org/issues/show_bug.cgi?id=115293 it contains NPE call stack as attachment for JDK 6u2.