better error message for polytopes without constraints

src/Interfaces/AbstractPolytope.jl

@@ -119,7 +119,14 @@ constraint of `P`.
 """
 function isuniversal(P::AbstractPolytope{N}, witness::Bool=false
                     )::Union{Bool, Tuple{Bool, Vector{N}}} where {N<:Real}
-    return witness ? isuniversal(constraints_list(P)[1], true) : false
+    if witness
+        constraints = constraints_list(P)
+        @assert !isempty(constraints) "polytopes without constraints are not " *
+            "allowed"
+        return isuniversal(constraints[1], true)
+    else
+        return false
+    end
 end
 
 # given a polytope P, apply the linear map P to each vertex of P