I have recently seen a fan-theory that in The Force Awakens,
was the one that activated Kylo Ren's lightsaber, with suicidal effect.
Is there a reliable (preferably canon) explanation of whether this fan-theory is true or false?
The film's official novelisation categorically disproves this theory
Then, as the light from outside was fully blocked by the flow of descending, accumulating dark energy, Ren ignited the lightsaber — and the fiery red beam lanced outward to pierce Han’s chest from front to back.
The junior novelisation contains a very slightly different view of the event.
Ben unbelted his lightsaber hilt and held it out. Han looked at the hilt and then at his son. Finally, he reached for it.
“Thank you,” Ben said. He ignited the lightsaber.
At first, Han didn’t feel anything except a piercing heat in his chest.
As does the film's script
Han actually smiles -- and reaches out for the dark weapon -- but with the light now gone, KYLO REN'S EYES FILL WITH DARKNESS, HE IGNITES THE LIGHTSABER -- THE FIERY BLADE SHOOTS OUT, RIGHT THROUGH HAN'S CHEST AND BACK!
as does The Force Awakens: Rey's Story
Kylo unholstered his lightsaber and slowly raised the hilt toward Han. He was close to his father. Rey saw the metal of the lightsaber flash in the dying sunlight. Han reached out to take the weapon.
But Kylo suddenly ignited the blade, and Han fell, lifeless, to the chasm below.
As does The Force Awakens: Finn's Story
From high above the catwalk, Finn saw Kylo drop his helmet, then offer his weapon to his father just as the last ray of sunshine disappeared from the sky outside.
But instead of letting Han take the weapon, Kylo ignited the blade.
As I recall, right before the fatality, Han was telling Kylo to come home, and offered to help, and that this help could be "anything". This was an appropriate offer, because Kylo had a lot of baggage that would need to be handled. Han's son took the available offer as an offer to help the son accomplish whatever the son decided he needed. Prior to the lethal extension, Han probably thought Kylo's responses were seeming rather promising for Han's wishes, which were to follow the orders of Kylos' mother: bring him home.
If Han was to fail in his mission to bring his son home that day, his fallback preference would probably have been to wait another day, and pursue Kylo again at a later time. Suiciding would be to giving up on that goal, which does not seem to be what Han was doing.
So, that fan-theory seems false, based just on my interpretation of the content of the movie.