Don't attempt to resize unmounted graphs

[wbrgss]

Simple check for a graph element before a resize attempt. Fixes #534.

[alexcjohnson]

We probably should remove the resize listener in componentWillUnmount as well. I'm imagining a case where you unmount and remount a plot many times, and we'll end up with a million resize listeners. Plotly.Plots.resize should be smart enough to not actually redraw the plot more than once, but there's still some overhead (and I wouldn't be surprised if there's a case where it would do many redraws).

Do we need this same conditional around the resize and bindEvents in plot too?

dash-core-components/src/components/Graph.react.js

Lines 114 to 115 in 6d945c4

	this.bindEvents();
	Plotly.Plots.resize(document.getElementById(id));

Much less likely, but in principle the plot element could be removed before it finishes drawing and enters that .then

[wbrgss]

probably should remove the resize listener in componentWillUnmount as well.

In order to do this I moved the resize handler from its anon function into a named function bound in the constructor.

[alexcjohnson]

Looks great, thanks! Given that this is a bugfix, would you mind locking it down with a test?

Also changelog

I'm 99% convinced the way you did the removeEventListener is right - was worried that we were referencing a class attr instead of an instance attr but I guess this.graphResize = this.graphResize.bind(this); ensures it's an instance attr and we're doing the right thing.

[wbrgss]

I added an integration test.

Before the changes in this PR (failing test):

======================================================================
ERROR: test_unmounted_graph_resize (test.test_integration.Tests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/plotly/Workspace/dash-core-components/test/test_integration.py", line 138, in test_unmounted_graph_resize
    raise Exception('browser error logged during test', entry)
Exception: ('browser error logged during test', {'level': 'SEVERE', 'message': 'http://localhost:8050/_dash-component-suites/dash_core_components/plotly-1.47.0.min.js?v=0.48.0&m=1555705071 6:2043297 Uncaught Error: DOM element provided is null or undefined', 'source': 'javascript', 'timestamp': 1560296987995})

----------------------------------------------------------------------
Ran 1 test in 5.934s

After:

.
----------------------------------------------------------------------
Ran 1 test in 5.954s

OK

(no exceptions raised)

[alexcjohnson]

Love it. Thank you! 💃