The background/context of this issue is a D7 to D8 migration.
A function was defined on the D7 side to return the Allowed Values for a multi-select field.
But the function did not exist (yet) on the D8 side.
Resulting in a White Screens of Death.
A more helpful outcome would be where the code "limps on" to the next page, so that the damage can be assessed.
The attached patch establishes this.
Needs work
9.5
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
This error also happens when calling a function of a disabled module, patch #2 solves it, since it first validates if the function exists.
Or is there another way to check this so that it doesn't call a function of a non-installed module?