src/Mutator/ReturnValue/This.php
public static function getMutation(array &$tokens, $index)
    {
        $tokenCount = count($tokens);
        for ($i=$index+1; $i < $tokenCount; $i++) {
            if (is_array($tokens[$i]) && $tokens[$i][0] == T_WHITESPACE) {
                continue;
            } elseif (is_array($tokens[$i]) && $tokens[$i][0] == T_VARIABLE && $tokens[$i][1] == '$this') {
                $tokens[$i] = [
                    T_STRING,
                    'null'
                ];
                break;
            }
        }
    }
public static function getMutation(array &$tokens, $index)
    {
        $tokenCount = count($tokens);
        for ($i=$index+1; $i < $tokenCount; $i++) {
            if (is_array($tokens[$i]) && $tokens[$i][0] == T_WHITESPACE) {
                continue;
            } elseif (is_array($tokens[$i]) && $tokens[$i][0] == T_VARIABLE && $tokens[$i][1] == '$this') {
                $tokens[$i] = [
                    T_STRING,
                    'null'
                ];
                break;
            }
        }
    }
Replace (return $this;) with (return null;)

@param array $tokens
@param int $index
@return array