title: Why gdspy library give a tkinter tcl error when trying to use the layout viewer
body: <p>I'm using the gdspy library which uses tkinter to show a layout preview.
  But when trying to use:</p>
  <pre><code>gdspy.LayoutViewer(library=None, cells=[cell])
  </code></pre>
  <p>python give me a:</p>
  <pre><code>    Traceback (most recent call last):
    File &quot;.\InductorGen.py&quot;, line 726, in &lt;module&gt;
      gdspy.LayoutViewer(library=None, cells=[cell])
    File &quot;C:\Users\Maël\Anaconda3\lib\site-packages\gdspy\viewer.py&quot;, line 182, in __init__
      self.grid(sticky='nsew')
    File &quot;C:\Users\Maël\Anaconda3\lib\tkinter\__init__.py&quot;, line 2226, in grid_configure
      + self._options(cnf, kw))
  _tkinter.TclError: cannot use geometry manager grid inside . which already has slaves managed by pack
  </code></pre>
  <p>here is the code from the layout viewer of gdspy: <a href="https://github.com/heitzmann/gdspy/blob/master/gdspy/viewer.py" rel="nofollow noreferrer">gdspy_layoutviewer</a></p>