Thanks @akikoskinen for reporting this.
Could you provide a reproducer for this similar to that one? #623

Here are files to reproduce this. I took an existing test file as a base, slightly modified to suit this case (the filenames have a .txt extension, although they are JSON files, since GitHub doesn't accept files with a .json extension). This realm file adds an identity provider with a custom first (and post, I'll return to this later) login flow.

00-initialize.json.txt

In the next step the identity provider's first login flow is reset to the Keycloak's built-in "first broker login" authentication flow and the custom authentication flow ("my custom first login flow") is removed.

01a-reset-first-login-flow.json.txt

Applying this realm change causes the error and is all for reproducing the original bug reported in this issue.

But I found out that this same issue also concerns the identity provider's post login flow. The 00-initialize.json file also sets a custom post login flow for the identity provider. If in a second step the post login flow is reset (this time to an empty value, which removes any post login flow from the identity provider) and the custom authentication flow is removed, the same error occurs.

01b-reset-post-login-flow.json.txt

Applying these realm files don't need any special options to be used. I used something like

java -jar ./keycloak-config-cli-19.0.3.jar --keycloak.url=http://localhost:8080/auth --keycloak.user=admin --keycloak.password=admin --import.files.locations=./00-initialize.json