Hi Kelvin;
I have PB apps back to version 1.0 and in-between. I use these to test every new Appeon PB migration from these lower PB versions.
First of all before you migrate anything to PB2019Rx, please run the "Migration" utility in PB 2019 on your PB 7 Application libraries (PBLs) to see if there are changes to the PB 7 App code that need to be done before you migrate in the PB 7 IDE.
Next, "optimize" all your PBLs in the PB 7 IDE. The PB IDE has a habit of leaving "ghost" object definitions in a PBL. The optimize step will remove these so that they do not accurately inhibit the PB 2019 migration step.
The next step is to copy all of your PB 7 App to a new folder. Never migrate a "live" version of any app in case this step blows up & you cannot recover your app in any PB version.
The next step is to create a new "Workspace" object from the PB 2019 IDE into your new copied PB 7 App folder.
The final step is to then create a new Native Application Target but make sure that you choose the "Existing" PB 7 Application, and it's existing PBLs. This should then have the PB 2019 prompt you to migrate. Answer yes and then let the IDE complete this step. Once the migration step is complete, you should now be able to view your App's source code. HTH
Regards ... Chris
Sounds like you have made good progress. You should now be at least able to look at the App's source code in the PB2019 IDE!
For the current error(s) you described, it sounds like the object that you are trying to open has an ancestor that is also in error and thus cannot be opened as well. Check out the ancestor that its referring to and then try a "regenerate" on that ancestor in the System Tree using the mouse RMB. Once the ancestor is OK, then the object you were initially trying to open should now also open.
Note: You can always look at the source code via the "Edit Source" option for any object class.
Regards ... Chris