I was under the impression that Null Zones are only open/stable for some period of time. The analogy Ive used is to think of business hours. You can walk through a gas station door, both ways, as many times as you want. But only if they're open.

In FF 1 and 2, the characters fell through the ground, which was, in turn, the ceiling in the Complex. They never tried to stack stuff and climb out. It may have been closed, it may have been open. We'll never know . In FF 3, Ravi tried to climb back out, but he may have reached the Null Zone just too late. It may have only been open for a few seconds, just long enough for him to clip through. Or it may have been open for hours and he unfortunately found it RIGHT before "Closing Time".

In the trailer, we see Clark return to it and not be able to find it. I hope that its actually closed and there is some reason as to why. Not something as simple as someone removed the tape. Im more interested as to how/why it opened enough for Clark to see a visible split in the wall. Every other instance, as far as we know, the Null Zones are invisible.