You have defined your function to have an input - th. Therefore, when you use your function handle, you have to supply it an input. Perhaps you meant to do the following?
x = fhandle(th).*cos(th);
The error message about 'cell' is because you add your function handles to a cell array, fset. When you use parentheses to index a cell array, it returns a cell array. Use braces to pass the function handle into the function.
[x,y] = my_polar(fset{i}, 10);
Your final error will be about trying to create a title from the function handle. You might find this answer insightful. You probably want to use func2str. Just be aware that it also does not work with cells.
