Why am I getting this message when I try to save in FMB?

[major_setback]

I found it was a fuel truck. I don't know why it required a waypoint.

Now I have another issue though. I get this message when trying to save (and it won't save):

building '0_bld' has unlinked hooks

...and when I delete this building I get the same message for '5_bld' etc.

I'll soon have nothing left of my mission objects. I deleted all trucks and now the buildings are going too.