Bridging cavity wall at floor level to patio door

[Dillz]

Hi,
I am shortly going to get a wooden floor installed and want the floor to go up to a patio door I have. At the moment there is a piece of stained wood screwed to the floor in front of the door, which I have now removed. Underneath is a length of damp proof course on top of the interior wall, then a cavity, then the patio door fixed into the exterior wall.

How do I fill the gap?

Can I fill with concrete without the risk of damp as it below the DPC?

Would it be a good idea to line the cavity with a damp proof membrane before I fill it with concrete?

Any advice greatly appreciated.
Thanks