Is the Karma theory an essential component of Buddha's teaching?
There is no Buddhism without Karma. Paticcasamuppada (the Law of Dependent Origination) is all about Karma and it's consequences. 

Akiriya-ditthi is the false view or opinion of the inefficacy of action: That neither moral good nor moral evil action have any delayed consequences for anyone.

This wrong view was taught by Pūrana-Kassapa during the time of the Buddha.

Today also there are people like Pūrana Kassapa who spread such views. What is more dangerous is that instead of presenting such views as their own beliefs, they try to present them as the teachings of the Buddha and mislead many who are new to Buddhism.