title: Zend_Db: Adapter suddenly is null!
body: <p>I'm using Zend_Db in one of my projects. 
  Now I have the problem, that suddenly during the code execution, the variable $_db in Zend_Db_Adapter_Abstract is null. (shown by var_dump($this); in my DbTable_xx class).</p>
  
  <p>It seems like the adapter is set to null somewhere during the script execution.
  How can that happen?</p>
  
  <p>Unfortunately the project is too complex to post some code here... I'm getting this error (while executing the find($primary) method on Zend_Db_Adapter_Abstract):</p>
  
  <pre><code>Fatal error: Call to a member function quoteTableAs() 
  on a non-object in xxx/library/Zend/Db/Table/Abstract.php on line 1162
  </code></pre>