Unable to submit workflow when non form root data source is used as workflow data source

If your workflow data source is NOT the form root data source, then when you submit the workflow you might encounter the following error:-

Wrong argument types in variable assignment.

The variable assignment error is due to different record is received by the SubmitToWorkflow class rather than the specified workflow record.

Although the WorkflowDatasource property in the form has been specified properly, the SubmitToWorkflow class will always use the form root data source as the workflow data source.
After debugging, I realised that the problem actually lies in the SysWorkflowFormControls class\ initControls() method. The line below

actionBarSubmitButton.dataSource(workflowDatasource.name());

is not working properly to assign the workflowDatasource to the workflow Submit button. Hence the Submit button treats its DataSource property is empty and use the root data source.

I created a quick test to add the SubmitToWorkflow menu item button manually to the form and use the above code to assign the data source. It was still not working.
Then I created another SubmitToWorkflow menu item button, but instead of using code, I specify the data source for the button using AOT Properties window. And this time the workflow was submitted.
So I can confirm that the MenuItemButton.dataSource() method did not do its job properly.

So what’s the workaround? If the DataSource property in MenuItemButton is empty, it will first use the DataSource specified in its parent container (i.e. Group control), and continue search upward till the DataSource specified in the form Design node. If no data source found, it will use the form root/first data source by default.
So to fix the workflow problem, we can use the above behaviour to implicitly assign the data source to the SubmitToWorkflow button. One way to do this is in the SysWorkflowFormControls class\ initControls(), add the below line

actionBarGroup.dataSource(workflowDatasource.name());

to assign the data source to group control that contains the button, which will then inherited by the button.