Dealing with nulls, dynamic columns in pivot

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SQL

Hi have nulls in data set that I'm trying to pivot with a total column and hence my total column is coming out as null. My code is as follows

DECLARE @cols       NVARCHAR(MAX)=''
DECLARE @query      NVARCHAR(MAX)=''
DECLARE @COLS_SUM   NVARCHAR(MAX)=''

--Preparing columns for Pivot
SELECT @cols = @cols + QUOTENAME(finmonth) + ',' 
FROM (SELECT DISTINCT finmonth FROM [DATASOURCE] ) AS tmp
SELECT @cols = SUBSTRING(@cols, 0, LEN(@cols))

--Preparing sum of columns for Totals Horizontal
SELECT @COLS_SUM = @COLS_SUM + QUOTENAME(finmonth) + '+' 
FROM (SELECT DISTINCT finmonth FROM [[DATASOURCE] ) AS tmp
SELECT @COLS_SUM = ','+ SUBSTRING(@COLS_SUM, 0, LEN(@COLS_SUM)) +' AS TOTAL'


SET @query = 
            'SELECT *'+@COLS_SUM+'  INTO #TAB FROM 
            (
                    SELECT var1, var2, var3, finmonth, sum(Actual_Activity) as [activity]
  FROM [[DATASOURCE]
  where [FinYear] = ''2018''
    group by  var1, var2, var3, finmonth
            ) src
            PIVOT 
            (
                   SUM([activity])  FOR finmonth IN (' + @cols + ')
            ) piv

            SELECT * FROM #TAB
            

            '

execute(@query)

Here is some example dummy data which would result in the issue. So where one Var1 (var1=a in below) has data for finmonth=2 but another doesn't. Var1 = b will have null for finmonth = 2 as they have no data for this month.

var1	var2	var3	finmonth	Actual_activity
a	b	c	1	20
a	b	c	1	30
a	b	c	1	40
a	b	c	2	10
a	b	c	2	20
a	b	c	2	30
b	b	c	1	1
b	b	c	1	2
b	b	c	1	3

The results table looks like this - but total for 2nd row should be 6

r1	var2	var3	1	2	Total
a	b	c	90	60	150
b	b	c	6	Null	Null

What I have tried:

I've tried using isnull in the main group by query but seems to have no effect