Bug fixing

[joaompneves]

Several bug fixes:

• Fixed null pointer exception after disposing webview
• Fixed memory leak after unloading control
• Replaced TryGetCefBrowser with GetCefBrowser in some methods to throw exception instead of ignoring the action

[perlun]

Hi,

This one I am more positive towards incorporating. I will just have to review it a bit more on my behalf first, to ensure that everything works as expected with these changes in place.

Thanks for your valuable contribution(s) to the project!

[perlun]

Sorry for the delay with this one. Would be good to get merged; we just have to weight it compared to trying to get CefSharp3 out the door...

[perlun]

Finally, started looking at this. The whitespace is a bit messed up (probably using tabs instead of spaces). Could you fix it?

[amaitland]

Firstly thank you @joaompneves for the fine contribution! As we've released the first official version of CefSharp3 and there is nobody actively working on developing CefSharp1 I'm going to close this PR. If someone wishes to step-up and take over CefSharp1 maintenance and development then this can be reopened.