Fatal Error in wp_get_current_user() Function

You call this function too early, before pluggable function is_user_logged_in() is defined. Try this:

function wpse_get_current_user() {
  $current_user = wp_get_current_user();
  // do other stuff here
}
add_action( 'init', 'wpse_get_current_user', 10 );