title: Error on a unique non-clustered index
body: <p>I am trying to move a field do a different position in the table (I'm just trying to get it in alphabetical order). When I click save, I get this error message (TaskID is the name of another field):</p>
  
  <pre><code>'tblTask' table
  - Unable to create index 'idx_TaskID_notnull'.  
  The CREATE UNIQUE INDEX statement terminated because a duplicate key was found for the object name 'dbo.tblTask' and the index name 'idx_TaskID_notnull'. The duplicate key value is (&lt;NULL&gt;).
  The statement has been terminated.
  </code></pre>
  
  <p>I have one unique non-clustered index for TaskID that has the filter <code>TaskID IS NOT NULL</code>.</p>
  
  <p>Why am I getting this error?</p>