WPF - InternalDispose add browser null check

It's possible to Dispose the browser before OnAfterBrowserCreated is called
resulting in a NRE when attempting to set WasHidden(true).

Resolves #3037

Author: amaitland

CefSharp.Wpf/ChromiumWebBrowser.cs

@@ -698,7 +698,8 @@ private void InternalDispose(bool disposing)
 
                 //Stop rendering immediately so later on when we dispose of the
                 //RenderHandler no further OnPaint calls take place
-                browser.GetHost().WasHidden(true);
+                //Check browser not null as it's possible to call Dispose before it's created
+                browser?.GetHost().WasHidden(true);
 
                 UiThreadRunAsync(() =>
                 {