When a SCM plugin is uninstalled, projects using it should fall back to "No SCM".

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@17300 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/model/AbstractProject.java

@@ -195,6 +195,8 @@ public abstract class AbstractProject<P extends AbstractProject<P,R>,R extends A
             triggers = new Vector<Trigger<?>>();
         for (Trigger t : triggers)
             t.start(this,false);
+        if(scm==null)
+            scm = new NullSCM(); // perhaps it was pointing to a plugin that no longer exists.
 
         if(transientActions==null)
             transientActions = new Vector<Action>();    // happens when loaded from disk