Throw error for empty search area

Author: pjaap

src/helper_functions.jl

@@ -355,6 +355,13 @@ function _get_periodic_coupling_matrix(
         append!(searchareas, view(faces_to, 1:nfaces_to))
     end
 
+    # throw error if no search area had been found for a bface
+    for source in 1:num_sources(searchareas)
+        if num_targets(searchareas, source) == 0
+            throw("bface $source has no valid search area on the opposite side of the grid. Are from/to regions and give_opposite! function correct?")
+        end
+    end
+
     # we are only interest in global bface numbers on the "from" boundary
     bfaces_of_interest = filter(face -> boundary_regions[face] in b_from, 1:n_boundary_faces)
     n_bface_of_interest = length(bfaces_of_interest)