231945: guard on null and tell them what to do about it

weaver/src/org/aspectj/weaver/tools/WeavingAdaptor.java

@@ -275,6 +275,12 @@ public class WeavingAdaptor implements IMessageContext {
 	 * @exception IOException weave failed
 	 */
 	public byte[] weaveClass(String name, byte[] bytes, boolean mustWeave) throws IOException {
+		if (trace==null) {
+			// Pr231945: we are likely to be under tomcat and ENABLE_CLEAR_REFERENCES hasn't been set
+			System.err.println("AspectJ Weaver cannot continue, static state has been cleared.  Are you under Tomcat? In order to weave '"+name+
+					"' during shutdown, 'org.apache.catalina.loader.WebappClassLoader.ENABLE_CLEAR_REFERENCES=false' must be set (see https://bugs.eclipse.org/bugs/show_bug.cgi?id=231945).");
+			return bytes;
+		}
 		if (weaverRunning.get()) {
 			// System.out.println("AJC: avoiding re-entrant call to transform " + name);
 			return bytes;