title: mobx-state-tree Error while converting `""` to `AnonymousModel[]`
body: <p>I am getting this error but I don't know why. I am running mobx state tree (3.2.4)</p>
  
  <pre><code>:8080/main.js:52013 Uncaught (in promise) Error: [mobx-state-tree] Error while converting `""` to `AnonymousModel[]`:
  
      value `""` is not assignable to type: `AnonymousModel[]` (Value is not an array).
      at fail (:8080/main.js:52013)
      at typecheck$$1 (:8080/main.js:51692)
      at typecheckInternal$$1 (:8080/main.js:51678)
      at ArrayType$$1../node_modules/mobx-state-tree/dist/mobx-state-tree.module.js.ArrayType$$1.applySnapshot (:8080/main.js:52884)
      at executeAction (:8080/main.js:55058)
      at ArrayType$$1.res (:8080/main.js:55050)
      at :8080/main.js:50844
      at executeAction (:8080/main.js:55058)
      at res (:8080/main.js:55050)
      at runMiddleWares (:8080/main.js:51533)
  </code></pre>
  
  <p>I cannot see any indication I am trying to set an empty string to an array.</p>
  
  <p>How can I pinpoint what property it is trying to apply?</p>