title: mps grammar cells giving message "Error Missing Dependency on jetbrains.mps.lang.core"
body: <p>I am following along Kolja Dummann's Heavy Meta (4) livestream on Youtube.</p>
  <p>I am trying to use the grammar cells to wrap a binary expression.  the projection looks like this</p>
  <pre><code>&lt;default&gt; editor for concept BinaryExpression                                                                                                          
    node cell layout:                                                                                                                                    
      projection: [&gt; wrap % left % grammar.constant wrap % right % &lt;]                                                                                    
      grammar: rule: &lt;derive from projection&gt;    (instance of CellModel_RefNode)  (instance of GrammarConstantQueryCell)  (instance of CellModel_RefNode)
                                                                                                                                                         
    inspected cell layout:                                                                                                                               
      &lt;choose cell model&gt;                                                                                                                                
                                                                                                                                                         
  </code></pre>
  <p>the <code>grammar.constant</code> <code>constant text</code> property is set to <code>subconcept.conceptAlias</code>.</p>
  <p>Unfortunately, I am getting an error message in the editor for both the wrap cells that says:</p>
  <pre><code>Error Missing Dependency on jetbrains.mps.lang.core
  </code></pre>
  <p>When I rebuild the editor, the language or the project, it reports that the rebuild is successful, however, I am unable to get the expected behavior of the aliases (+,-,*,/) being recognized after I type a number in the sandbox project.</p>
  <p>I have added the jetbrains.mps.lang.core to the editor, but it shows up greyed out, i.e. unused.</p>
  <p>when earlier in the tutorial I used <code>wrap</code> on my number literal editor, the expected behavior happened even though I got the editor showed this same error message. so perhaps the error message is a red herring.</p>
  <p>I am using</p>
  <ul>
  <li>MPS 2020.2 (202.6397.948). (I had same issue in 2020.1)</li>
  <li>the grammarcells from de.itemis.mps.extensions-2020.1.1477.e4af43c (I think - not sure how to check).</li>
  </ul>