Calling setStyle with a style url throws TypeError: Cannot read property 'send' of undefined

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**mapbox-gl-js version**:
0.45.0

**browser**:
Chrome

I really like the `setStyle` and `diffStyles` approach for React + MapboxGL integration outlined in [this article](https://hackernoon.com/react-redux-with-mapbox-78fa3767211e), but I am noticing a possible bug when calling `map.setStyle` with default mapbox style urls.

### Steps to Trigger Behavior

1. Instantiate a map using a default `mapbox://` style url
2. Wait for the `style.load` event, then call `setStyle` with a new style url
3. Repeat Step 2 quickly.

Following these steps slowly (see gif) works fine, but going quickly causes an error to throw:
```
TypeError: Cannot read property 'send' of undefined
e.P.send https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:32:15431
i.loadTile https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:32:21086
i._loadTile https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:32:55673
i._addTile https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:32:62499
i._updateRetainedTiles https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:32:61509
i.update https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:32:60859
i.eval https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:32:54448
i.D.fire https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:28:8423
i.D.fire https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:28:8742
eval https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:32:19998
o https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:32:15051
XMLHttpRequest.r.onload https://qx91nqjlyj.codesandbox.io/node_modules/mapbox-gl/dist/mapbox-gl.js:28:251128
```

### Link to Demonstration

In this demo, I disable the buttons which trigger calls to `map.setStyle` until the `style.load` event fires.

https://codesandbox.io/s/qx91nqjlyj


### Expected Behavior

Calling `map.setStyle` should not throw an error if it is done after the `style.load` event fires.

### Actual Behavior

An error gets thrown by calls to `map.setStyle` even after waiting for `style.load` to fire first.

Start with the linked CodeSandbox and reproduce rapid map.setStyle calls after style.load fires. Trace the setStyle path to the failing send call and determine why it becomes undefined during repeated style changes. Done means repeated style URL changes no longer throw the reported TypeError.

Written by the indexing model from the issue text.