Cannot Complete Task When Document is Open

[#1]

In our Policies and Procedures workflow, we have many reviewers assigned the same Flexi Task.  When a Reviewer is finished reviewing, when they try to complete their task, they receive an error if they have the document open or if it is opened by another Reviewer.  How can this be resolved?  I found the following thread referring to the same issue:  https://community.nintex.com/thread/1354  However, the solution entered there will not work for us.  Reviewers have to actually edit the document, so an attachment to the Task Notification will not work.

[burkslm]

I recommend using a Checkout action and then using a Checkin action when the workflow is complete.

[#3]

Like this?

I'm unsure how that will that work when there are multiple users assigned to the Flexi Task?  Should I turn on 'Check In / Check Out' at the SharePoint Library level instead?  Your help is greatly appreciated!!