Add a null check before trying to read what error was detected.

Fixes #29.

Author: Clorith

class-jquery-migrate-helper.php

@@ -286,7 +286,7 @@ public static function fatal_error_handler() {
  var erroredFunction = msg.match( match_pattern );
 
  // If there was no matching functions, do not try to downgrade.
- if ( typeof erroredFunction !== 'object' || typeof erroredFunction[1] === "undefined" || -1 === jQueryFunctions.indexOf( erroredFunction[1] ) ) {
+ if ( null === erroredFunction || typeof erroredFunction !== 'object' || typeof erroredFunction[1] === "undefined" || -1 === jQueryFunctions.indexOf( erroredFunction[1] ) ) {
  return true;
  }