What if I do not feel safe to jump on the line without mattress?
Instability is part of the game, especially at the beginning! To make progress, if you prefer, you can put some gym mattresses below the line.
If you are afraid of falling off your Trickline, you should learn the dismount trick, placing your hand on the ground in the swing low point (starting from the sitting position, the butt bounce). This simple movement, once mastered, will allow you to save yourself in the worst situations! Remember that before learning how to jump, it is important to learn how to fall safely.