When we implemented OSP target support, we made a mistake (in #812) in the logic that detects whether the user needs to be warned that there is no conversion host configured.
Currently, if there are no OSP conversion hosts anywhere the appliance can find, the UI displays a warning on the clusters step of the wizard reading "No OpenStack conversion hosts are configured. You can continue to create an infrastructure mapping, but you must configure a conversion host before migration execution."
However, this warning just checks whether OSP conversion hosts exist at all, it does not take into account which provider/EMS they reside in. If the user has more than one OSP provider configured, they could in theory have a conversion host in provider A (and therefore not see the warning), but create a mapping to provider B which would fail.
The conversion host warning for OSP needs to be similar to the one for RHV (a warning icon on each target cluster in the list), but instead of looking for the conversion host in that specific target cluster/project, it should look for a conversion host with the same EMS id as that target cluster/project.