When you define a footer for one column but skip the others, at first the grid seems to initialize and work as expected. However if u decide to show/hide a column it throws an error and may takes HOURS of debugging till u find the source of the error.
Unfortunately I was not able to reproduce your issue locally.
The following snippet works well for me:
https://snippet.dhtmlx.com/ia3xhwql
If the problem still occurs for your please, provide a complete demo or a demo link, where the problem can be reconstructed.