Fix NPE in PriorityDescriptorCache

Fixes #114

Author: adangel

ReleaseNotes.md

@@ -13,6 +13,8 @@ Eclipse Update Site:
 
 ### Fixed Issues
 
+*   [#114](https://github.com/pmd/pmd-eclipse-plugin/issues/114): NPE in PriorityDescriptorCache
+
 ### API Changes
 
 ### External Contributions

net.sourceforge.pmd.eclipse.plugin/src/main/java/net/sourceforge/pmd/eclipse/plugin/PMDPlugin.java

@@ -376,15 +376,17 @@ public void run() {
      * @see org.eclipse.ui.plugin.AbstractUIPlugin#stop(org.osgi.framework. BundleContext )
      */
     public void stop(BundleContext context) throws Exception {
-
         fileChangeListenerEnabled(false);
 
-        plugin = null;
         disposeResources();
         ShapePainter.disposeAll();
         ResourceManager.dispose();
         PriorityDescriptorCache.INSTANCE.dispose();
         logbackConfiguration.unconfigureLogback();
+
+        // only unset the plugin reference after all our own shutdown logic
+        // has been executed
+        plugin = null;
         super.stop(context);
     }