I have modified the tfs bug work item template as below so that while resolving a bug from active to resolved state root cause field is not empty.
AS Microsoft.VSTS.Actions.Checkin action is defined user can checkin in team explorer marking the work item as resolved.This cause error
Failed to update the following work item: ID 39169. Reason: TF237124: Work Item is not ready to save
How can i allow user to update the root cause field while check-in from team explorer pend changes window
<TRANSITION from="Active" to="Resolved">
<REASONS>
....
</REASONS>
<FIELDS>
....
<FIELD refname="RootCause">
<REQUIRED />
</FIELD>
</FIELDS>
<ACTIONS>
<ACTION value="Microsoft.VSTS.Actions.Checkin" />
</ACTIONS>
</TRANSITION>