Incorrect iteration over tokens lead to error, code example:
$language = new ExpressionLanguage();
$parsedExpression = $language->parse('node.', ['node']);

Expected result: SyntaxError('Unexpected end of expression around position 5')
Actual result: Notice: Undefined offset: 3

Error can be found at Symfony\Component\ExpressionLanguage\TokenStream:58
Lack of checking the key exist lead to this error