MSQL error - #1248 - Every derived table must have its own alias

Question

I made this query with reference to this blogI am getting an error while using this query in MySQL.

The query logic is correct but I'm getting an error when running this in MySQL.

I looked at previous questions on StackOverflow, but didn't find something to help me.

Here is the query:

SELECT
COUNT(Button) AS Button,
COUNT(BurgetMenu) AS BurgerMenu,
COUNT(Tabs) AS Tabs,
SUM(CASE WHEN Button <= BurgerMenu THEN 1 ELSE 0 END) AS conversion_1_2,
SUM(CASE WHEN Button <= BurgerMenu AND BurgerMenu <= Tabs THEN 1 ELSE 0 END) 
AS conversion_2_3

FROM (SELECT 
     user_id,
     MAX(CASE WHEN `event_type` = 'Button' THEN first_occurance ELSE NULL END) AS Button,
     MAX(CASE WHEN `event_type` = 'BurgerMenu' THEN first_occurance ELSE NULL END) AS BurgerMenu,
     MAX(CASE WHEN `event_type` = 'Tabs' THEN first_occurance ELSE NULL END)AS Tabs

  FROM (SELECT deviceid, `event_type`, `date_time` AS first_occurance
        FROM events
        WHERE `date_time` BETWEEN '2017-10-30' AND '2017-11-06'
           AND `event_type` IN('Button','BurgerMenu','Tabs')
           AND deviceid IS NOT NULL
        GROUP BY `deviceid`, `event_type`) a

  GROUP BY `deviceid`)

I know that Stackoverflow has many answers for same error , but do please help with my query, i have referred with previous answers