Accidental ignition during restoration works at Notre-Dame is still the main version of the ongoing investigation of the fire reasons.
But the fire at Notre-Dame began to storm too strongly and everywhere at the same time. This is not a gas station to flash from one spark. There wasn’t enough wooden elements to be embraced by such a powerful fire. Many experts doubt that the cause of the fire was natural.
The second version: revenge against Macron from the left groups of Yellow vests. The fire started directly before Emmanuel Macron was supposed to address the nation and he was forced to postpone his speech. In France there are a lot of ultra leftists of all colours, from Trotskyists to anarchists, perhaps this was a planned blow against everything that they hate: religion, state, and Macron as the formal leader.
All this, of course, makes the “Islamist” hypothesis much less probable. However, it occurred on the day of the beginning of the Holy Week in France. And there were already numerous cases when terrible events took place at this particular time. So, the “message” is evident.