How to remove a variable from a sheet?

Question

Unity

There are 4 bases in which enemies spawn from time to time, there can be a maximum of 5 enemies on the stage. The spawn of the object is implemented in an empty object (software) that unites all 4 bases, the bases can be destroyed.

So, in the software, the choice of the base in which the enemy will appear is implemented through a list of 4 objects, if the bases are not destroyed, but the enemies are destroyed, then the enemies regularly appear until a maximum of 5 enemies is reached, but if you destroy 1 base, then the enemies stop appearing at all. (Implemented via Bolt)

The last GetPosition is highlighted in red, because. the object has been destroyed and Bolt cannot find this object

. If you know how to fix it, you can also offer it in the form of C # code)))