Refuse to construct nil objects.

src/syntax.jl

@@ -195,17 +195,25 @@ macro objcwrapper(ex...)
       struct $name <: $super
         ptr::id
 
-        # restrict the default constructor
-        $name(ptr::id) = new(ptr)
+        # restrict the default constructor to object pointer types,
+        # and make sure we don't create nil objects
+        function $name(ptr::id)
+          ptr == nil && throw(UndefRefError())
+          new(ptr)
+        end
       end
     end
   else
     quote
       mutable struct $name <: $super
         ptr::id
 
-        # restrict the default constructor
-        $name(ptr::id) = new(ptr)
+        # restrict the default constructor to object pointer types,
+        # and make sure we don't create nil objects
+        function $name(ptr::id)
+          ptr == nil && throw(UndefRefError())
+          new(ptr)
+        end
       end
     end
   end

test/runtests.jl

@@ -20,12 +20,17 @@ using ObjectiveC
 end
 
 # smoke test
+@objcwrapper TestNSString <: Object
 let data = "test"
     ptr = @objc [NSString stringWithUTF8String:data::Ptr{UInt8}]::id
     @test ptr isa id
     @test class(ptr) isa Class
     @test "length" in methods(ptr)
     @test 4 == @objc [ptr::id length]::Culong
+
+    obj = TestNSString(ptr)
+    @test Base.unsafe_convert(id, obj) == ptr
+    @test_throws UndefRefError TestNSString(nil)
 end
 
 @testset "foundation" begin