geany/geany — comment on commit e53d5b82992550921451d30487b1a426477338f5
File: src/highlightingmappings.h line 93
By b4n:

Here it is needed because that constant may be (actually, is) used in G_N_ELEMENTS() macro which dereferences the pointer; and a void* pointer cannot be dereferenced.
But don't worry, the actual code that would dereference this will not be called, it's guarded with a NULL check.