Currently its GetWebContents method returns null. Instead it should return the webcontents of its webview, but this leads to other problems. From bug https://crbug.com/680329#c19:

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I'm still looking into returning the proper webcontents from SimpleWebViewDialog. Doing that revelead bug 698438 and other problems though: The location bar and omnibox code reference nonexistant objects which results in crashes. This is because there is no browser instance for SimpleWebViewDialog, and no TabsHelper, which creates some of these WebContentsUserData objects (ExtensionTabHelper, ChromeTranslateClient, ManagePasswordsUIController, FaviconDriver, SearchTabHelper etc). The question is, do we want to create these separately for SimpleWebViewDialog (1), or add null checks for them (2)? The advantage of (1) is that it contains all changes SimpleWebViewDialog. The better fix is obviously to move away from using SimpleWebViewDialog but that's a bigger change."