Displaying a previously created Matplotlib figure in a widget

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I'm trying to find a way to take a figure previously created, and show it inside a widget, _without losing interactivity._
Here's a short version of my approach (which may not be the right one):

```python
fig = plt.figure() # previously created figure
out = widgets.Output()
with out:
display(fig)
# other things happening
display(widgets.HBox([fig.canvas, some_other_fig]))
```

If I first create a figure in the notebook outside of an `Output` widget, and then wish to include it later inside something like an `HBox` next to a second figure, I seem to lose interactivity with the figure (including the nice ipympl zoom interface).

Here is a [nbviewer view](https://nbviewer.jupyter.org/github/thomasaarholt/tests/blob/master/Output%20Example%20Notebook.ipynb) of a MWE notebook I created, download it top right on the window. It's best if one downloads it and runs it since the first figure appears buggy in the nbviewer.

Additionally, and I might create a new issue for this - if I call `fig1` at the end of the notebook, I get an unexpected error. Here I'd expect the regular figure to be shown. The error is `AttributeError: 'NoneType' object has no attribute '_send_event'`.

Lade das verlinkte Output Example Notebook herunter und führe es aus, um das Verhalten mit der zuvor erstellten Abbildung, dem Output widget und HBox zu reproduzieren. Beginne damit nachzuverfolgen, wie die angezeigte Abbildung und ihr Canvas behandelt werden, und überprüfe dann, dass die Abbildung ihre ipympl-Interaktivität in HBox beibehält und dass das Anzeigen von fig1 nicht mehr den gemeldeten NoneType-Fehler auslöst.

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