title: Show call failing with a grouped and aggregated dataframe
body: <p>I am trying to use sum after groupBy, like this,</p>
  
  <pre><code>val b = a.groupBy($"key").agg(sum($"value"))
  </code></pre>
  
  <p>Here the schema of a is of the following type,</p>
  
  <pre><code> |-- key: string (nullable = true)
   |-- value: integer (nullable = false)
  </code></pre>
  
  <p>While the schema of b is of the following type,</p>
  
  <pre><code> |-- key: string (nullable = true)
   |-- sum(value): long (nullable = true)
  </code></pre>
  
  <p>But when I do <code>b.show</code>, I get this error.</p>
  
  <pre><code>cannot assign instance of scala.collection.immutable.List$SerializationProxy 
  to field org.apache.spark.rdd.RDD.org$apache$spark$rdd$RDD$$dependencies_ 
  of type scala.collection.Seq in instance of org.apache.spark.rdd.MapPartitionsRDD
  </code></pre>
  
  <p>What could be the reason for this error? I am using Spark 2.3.2 and running the code with an Apache Zeppelin note.</p>