Add a more descriptive error in case of fetch response not returning back what it should.

- An example of this is when a Promise library is being used and the Promise can not be identified correctly.  Thus the internal error.

1 files changed, 3 insertions, 1 deletions

diff --git a/sdks/wasm/framework/src/WebAssembly.Net.Http/WasmHttpMessageHandler.cs b/sdks/wasm/framework/src/WebAssembly.Net.Http/WasmHttpMessageHandler.cs
index 8296becf674..23334f46a26 100644
--- a/sdks/wasm/framework/src/WebAssembly.Net.Http/WasmHttpMessageHandler.cs
+++ b/sdks/wasm/framework/src/WebAssembly.Net.Http/WasmHttpMessageHandler.cs
@@ -146,8 +146,10 @@ namespace WebAssembly.Net.Http.HttpClient {
 
 				requestObject.Dispose ();
 
-				var response = (Task<object>)fetch.Invoke ("apply", window, args);
+				var response = fetch.Invoke ("apply", window, args) as Task<object>;
 				args.Dispose ();
+				if (response == null)
+					throw new Exception("Internal error marshalling the response Promise from `fetch`.");
 
 				var t = await response;