Fixed NullReferenceException on ControlExtensions.IsActiveControl call

[RomanDidenko]

Fixes NullReferenceException exception when browser control is removed from a form but not disposed as should be added back to the form at some point of time:

Description: The process was terminated due to an unhandled exception.
Exception Info: System.NullReferenceException
Stack:
   at CefSharp.WinForms.Internals.ControlExtensions.IsActiveControl(System.Windows.Forms.Control)
   at CefSharp.WinForms.Internals.ParentFormMessageInterceptor.WndProc(System.Windows.Forms.Message ByRef)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr, Int32, IntPtr, IntPtr)
   at System.Windows.Forms.UnsafeNativeMethods.PeekMessage(MSG ByRef, System.Runtime.InteropServices.HandleRef, Int32, Int32, Int32)
   at System.Windows.Forms.Application+ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr, Int32, Int32)
   at System.Windows.Forms.Application+ThreadContext.RunMessageLoopInner(Int32, System.Windows.Forms.ApplicationContext)
   at System.Windows.Forms.Application+ThreadContext.RunMessageLoop(Int32, System.Windows.Forms.ApplicationContext)
   at System.Windows.Forms.Application.Run(System.Windows.Forms.Form)
   at Lombard.UserInterface.Shell.Program.Main()

Windows 7, x86

[amaitland]

Great, thanks 👍

Looks straight forward, merging now 😄

[amaitland]

New package pushed to nuget.org

[Roman-Didenko]

Great, thanks 👍