title: Keeping a placeholder cell in a UITableView
body: <p>I have a UITableView that I never want to fall below 1 cell: It's a directory readout, and if there's no files in the directory, it has a single cell that says "No Files". (In edit mode, there's a bonus cell to Create a File, so edit mode never falls below two cells.)</p>
  
  <p>It's probably just lack of sleep keeping me from thinking my way out of a paper bag right now, but I keep tripping up on errors like this:</p>
  
  <pre><code>*** Terminating app due to uncaught exception 
  'NSInternalInconsistencyException', reason: 'Invalid update: 
  invalid number of sections.  The number of sections contained 
  in the table view after the update (2) must be equal to 
  the number of sections contained in the table view before 
  the update (2), plus or minus the number of sections inserted 
  or deleted (1 inserted, 0 deleted).'</code></pre>
  
  <p>This is happening as I'm preemptively adding the "No Files" placeholder before deleting the last file. There's a like crash if I delete the last file cell before adding the placeholder. Either way, the cell count gets out of sync with the return from numberOfRowsInSection, and that triggers a crash.</p>
  
  <p>Surely there's a design pattern for this situation. Clue me in?</p>