Added Vdb Plugin state check to prevent preview vdb jobs from being

processed on start-up

plugins/org.teiid.designer.dqp/src/org/teiid/designer/runtime/preview/jobs/WorkspacePreviewVdbJob.java

@@ -180,6 +180,9 @@ public boolean shouldRun() {
         // Want to avoid generating previews during vdb conversion
         // since models can be in partial states
         //
+
+        if( VdbPlugin.singleton() == null ) return false;
+
         if (result)
             result = ! VdbPlugin.singleton().conversionInProgress();