title: How to show Backtrace for emacs?
body: <p>I encounter an error:</p>
  
  <pre><code>scala-block-indentation: Wrong type argument: number-or-marker-p, nil
  </code></pre>
  
  <p>Whenever I click <kbd>tab</kbd> for indent in <code>scala-mode</code>.</p>
  
  <p>I don't know where goes wrong in the code but want to fix it. However, I can only see this error in <code>*Message*</code> buffer without details. I tried to see this error in Backtrace but didn't know how.  I tried (1) <code>-debug-init</code> to launch emacs (2) <code>debug-on-entry</code> command for <code>scala-block-indentation</code> but neither of them works.</p>
  
  <p>Does anyone have ideas about how to enable BackTrace/Debugger for this function?</p>