Notice: Trying to access array offset on value of type null

return $this->last_result[$y]?$this->last_result[$y]:null;

.....
.....
        function get_row($query=null,$output=OBJECT,$y=0)
        {
            // Log how the function was called
            $this->func_call = "\$db->get_row(\"$query\",$output,$y)";
            // If there is a query then perform it if not then use cached results..
            if ( $query )
            {
                $this->query($query);
            }
            // If the output is an object then return object using the row offset..
            if ( $output == OBJECT )
            {
                return $this->last_result[$y]?$this->last_result[$y]:null;
            }
            // If the output is an associative array then return row as such..
            elseif ( $output == ARRAY_A )
            {
                return $this->last_result[$y]?get_object_vars($this->last_result[$y]):null;
            }
            // If the output is an numerical array then return row as such..
            elseif ( $output == ARRAY_N )
            {
                return $this->last_result[$y]?array_values(get_object_vars($this->last_result[$y])):null;
            }
            // If invalid output type was specified..
            else
            {
                $this->show_errors ? trigger_error(" \$db->get_row(string query, output type, int offset) -- Output type must be one of: OBJECT, ARRAY_A, ARRAY_N",E_USER_WARNING) : null;
            }
        }
.....
.....

return $this->last_result[$y] ?? null;

isset ( $var[0] ) ?? $var[0] : null