A Hallucination isn’t the same as a delusion. A hallucination is a sensory experience that fires areas of the brain as if the person is actually seeing/hearing/feeling/tasting/smelling that literal thing. A delusion is a false belief. People can share delusions, they can actually “share” hallucinations, or both. There are several problems with this “theory” when applied to the resurrection.
First, if it was mass hallucination, it would have been an easy thing for either the Romans or the Jewish leaders to present the actual body.
Second (and this applies to Paul and the rest who saw Him post-resurrection), a hallucination or delusion is a disorder. Disorders are bad because they lead to negative outcomes over time. The witnesses, Paul, etc… did not have a breakdown of self or of functioning, they had a healing and a change in the positive direction.
Third, the normal conditions of shared psychosis are not present at the time of the resurrection. Jesus was seen by many people in many different contexts. The people were not socially isolated, they were throughout Jerusalem.