Object not set to an instance of an object error is generated when a dialog workflow component is reached. This issue only occurs when the project is published. The project works without issue when using debugger. Confirmed the Workflow service & the application pool are both using the same App Identity. All configurations are set to use the NetBIOS name for the server.
Project was created in Workflow 7.6 and runs fine. This issue was found when trying to move it to the Workflow 8.5 RU4 environment.
Release : Workflow 8.5 RU4
The fix for this issue is found in the following KB and it is the attachment with the name "Workflow_8_5_POST_RU4_v5_27July_2021_1627392380279.zip":
198337 CUMULATIVE POST ITMS 8.5 RU4 POINT FIXES
Note: "Workflow_8.5_POST_RU4_v5_27Jul_2021.zip" is a separate "not cumulative" point fix. It just contains some fixed dlls for different Workflow 8.5 RU4 released versions including 8.5.5710 version of previous "Workflow_8.5_POST_RU4_v2_26Nov.zip"