title: Replace all ":" with "_" in Spark dataframe
body: <p>I'm trying to replace all instances of ":" --> "_" in a single column of a Spark dataframe. I'm trying to do this with:</p>
  
  <pre><code>val url_cleaner = (s:String) =&gt; {
     s.replaceAll(":","_")
  }
  val url_cleaner_udf = udf(url_cleaner)
  val df = old_df.withColumn("newCol", url_cleaner_udf(old_df("oldCol")) )
  </code></pre>
  
  <p>But I keep getting the error:</p>
  
  <pre><code> SparkException: Job aborted due to stage failure: Task 0 in stage 25.0 failed 4 times, most recent failure: Lost task 0.3 in stage 25.0 (TID 692, ip-10-81-194-29.ec2.internal): java.lang.NullPointerException
  </code></pre>
  
  <p>Where am I going wrong in the udf?</p>