You define the following:
StackPanel topSP = null;
Then you have
StackPanel topSP = new StackPanel { Width = 400, Height = 50 };
This is in your local scope though so will be destroyed when you exit the function
Finally you have
topSP.Children.Add(myWrapPanel);
This will still be set to null which is why your error occurs. Basically you are creating a local version of the same variable.
In order to resolve simply change the second definition to:
topSP = new StackPanel { Width = 400, Height = 50 };