TypeError: Cannot read property '_childIndexes' of null

Suite and Optimus Suite 5
#1 
var parent = 1 //main row
var childNo = 2//mumber of child
var row = '';
var sumChild = 0;

for(var i = 1; i<=childNo; i++) {
    row = parent + '.' + i;
    sumChild += parseInt(myTreeGrid.cells(row,7).getValue());
}
myTreeGrid.cells(parent, 7).setValue(sum);// error : Uncaught TypeError: Cannot read property '_childIndexes' of null

myTreeGrid.cellById(parent, 7).setValue(sum);// error : Uncaught TypeError: Cannot read property '_childIndexes' of null

change data event onCellChanged then Calculate data in cell 7 all tree row for update data in main row cell 7 but error
Uncaught TypeError: Cannot read property ‘_childIndexes’ of null

Help me please!!! sir.

#2

Please, make sure that addressing cell is defined in the grid.
check if you have a row with ID=“1” column with the INDEX=“7”

If the problem still occurs for you please, provide with a complete demo or share with a demo link, where the problem can be reconstructed.