Android: WebView null exception fix

[m1ga]

JIRA:
https://jira.appcelerator.org/browse/TIMOB-26993

Note:
This issue was sometimes causing random crashes in our mocha unit test suite below. This solves that issue. And a new unit test is not needed.
https://github.com/appcelerator/titanium-mobile-mocha-suite/blob/master/Resources/ti.ui.webview.test.js

Test:

1. Build and run the below code on Android.
2. Tap on the "Open Child" button.
3. Verify that the WebView displays Google's website zoomed-in.
4. Tap the Android "Back" button.
5. Tap on the "Open Child" button again.
6. Verify that the WebView displays Google's website zoomed-in again.

var parentWindow = Ti.UI.createWindow();
var openButton = Ti.UI.createButton({ title: "Open Child" });
openButton.addEventListener("click", function() {
	var childWindow = Ti.UI.createWindow({ backgroundColor: "white" });
	var webView = Ti.UI.createWebView({
		url: "https://www.google.com",
	});
	webView.addEventListener("load", function() {
		webView.zoomLevel = 3.0;
		childWindow.close();
	});
	childWindow.add(webView);
	childWindow.open();
});
parentWindow.add(openButton);
parentWindow.open();

[build]

	Fails
🚫	🔬 There are library changes, but no changes to the unit tests. That's OK as long as you're refactoring existing code, but will require an admin to merge this PR. Please see README.md#unit-tests for docs on unit testing.

	Warnings
⚠️	There is no linked JIRA ticket in the PR body. Please include the URL of the relevant JIRA ticket. If you need to, you may file a ticket on JIRA

	Messages
📖	💾 Here's the generated SDK zipfile.
📖	🎉 Another contribution from our awesome community member, m1ga! Thanks again for helping us make Titanium SDK better. 👍
📖	✅ All tests are passing Nice one! All 2990 tests are passing.

Generated by 🚫 dangerJS against 0d7dc38

[garymathews]

CR: PASS

[build]

	Warnings
⚠️	There is no linked JIRA ticket in the PR body. Please include the URL of the relevant JIRA ticket. If you need to, you may file a ticket on JIRA

	Messages
📖	💾 Here's the generated SDK zipfile.
📖	🎉 Another contribution from our awesome community member, m1ga! Thanks again for helping us make Titanium SDK better. 👍
📖	✅ All tests are passing Nice one! All 3795 tests are passing. (There are 466 tests skipped)

Generated by 🚫 dangerJS against 9520ec6

[lokeshchdhry]

FR Passed.

No crash seen with the fix.

Studio Ver: 5.1.2.201903111843
SDK Ver: 8.1.0 local build
OS Ver: 10.14
Xcode Ver: Xcode 10.1
Appc NPM: 4.2.13
Appc CLI: 7.0.10
Daemon Ver: 1.1.3
Ti CLI Ver: 5.1.1
Alloy Ver: 1.13.9
Node Ver: 8.15.1
NPM Ver: 6.4.1
Java Ver: 10.0.2
Devices: ⇨ google Nexus 5 (Android 6.0.1)
⇨ google Pixel (Android 9)