title: Eclipse: All projects in the workspace start to give An internal error occurred during: "Validating XXXX" NullPointer Exception
body: <p>All of a sudden as I accidentally remove a jar file from maven repository .m2 folder(I am not sure it is relevant) and Eclipse complaint about this error message to all projects.</p>
  
  <p><img src="https://i.stack.imgur.com/I1elB.png" alt="enter image description here"></p>
  
  <p>How can I clean this up?</p>
  
  <p>I can see if I set STS/Eclipse as "Build Automatically" then if I make any change to the application then the project can rebuild the update, but if I choose to "clean.." and make clean build of the whole project then this error start to kick out.</p>