[Upgrade][XDCR][UI] Errors thrown while upgrading source cluster from 6.6.2 to 7.0.0

Build : 7.0.0-5016

Steps :
1. Setup 2x2 clusters running 6.6.2 with kv+n1ql+index+fts on both the nodes and install all 3 sample buckets - add a default bucket as well and pump in data
2. Setup replications between all the buckets
3. Upgrade target cluster to 7.0 (pump in data while cluster is in mixed mode)
4. Ensure data is all caught up
5. Start upgrading src cluster - pause in mixed mode

Check replications - errors observed in UI - attaching screenshots

Logs:
https://s3.amazonaws.com/bugdb/jira/upgradetesting/collectinfo-2021-04-23T215242-ns_1%40172.23.107.126.zip
https://s3.amazonaws.com/bugdb/jira/upgradetesting/collectinfo-2021-04-23T215242-ns_1%40172.23.107.142.zip