We have upgraded our project from AEM 6.4 to 6.5.6. We are getting blank screen when looked in workflow models Touch UI console.
All the workflow related bundles are active.
We can see all our custom and ootb workflow models in classic UI and the functionality also seemed to be working fine when triggered from Classic UI.
All our workflow models are present under either /conf or /var.
We are getting below error log when accessing workflow models from touch UI console.
26.10.2020 06:02:57.590 *ERROR* [184.108.40.206  GET /libs/cq/workflow/admin/console/content/models.html HTTP/1.1] org.apache.sling.engine.impl.SlingRequestProcessorImpl service: Uncaught SlingException
at org.apache.sling.scripting.jsp.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) [org.apache.sling.scripting.jsp:2.3.6]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:725) [org.apache.felix.http.servlet-api:1.1.2]
at org.apache.sling.scripting.jsp.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:502) [org.apache.sling.scripting.jsp:2.3.6]
In that case, if you used the Touch UI to open the "E-mailTest" workflow and click 'Edit', it would have created this /conf node -- you must do this to make changes and to have had the 'lastupdatedby' value change from default 'admin'.
You can resolve the issue by copying the node from /etc/workflow/models/E-mailTest and pasting it under /conf ( /conf/global/settings/workflow/models/E-mailTest).
Please ensure package is not causing deletion of the "design-time" version of the models .
26.10.2020 13:07:11.852 *WARN* [220.127.116.11  GET /libs/cq/workflow/admin/console/content/models.html HTTP/1.1] libs.cq.workflow.admin.console.components.item.workflowmodel.card.card$jsp Design time path not found for model with ID: /var/workflow/models/E-mailTest
Can you share complete error.log when you load the workflow model screen? To my understanding, it should also show some warning related to workflow model similar to  before throwing the given null pointer exception.
20.10.2020 18:21:14.269 *WARN* [18.104.22.168  GET /libs/cq/workflow/admin/console/content/models.html HTTP/1.1] libs.cq.workflow.admin.console.components.item.workflowmodel.card.card$jsp Design time path not found for model with ID: /var/workflow/models/avalara-page-activate
If you see similar logging like above "Design time path not found for model with ID", then that means the workflow model is not present in the design time path (/conf/global/settings/workflow/models).