fix nil-dereference panic

invoke.go

@@ -264,9 +264,11 @@ func invokeServerStream(ctx context.Context, stub grpcdynamic.Stub, md *desc.Met
 	// Now we can actually invoke the RPC!
 	str, err := stub.InvokeRpcServerStream(ctx, md, req)
 
+	if str != nil {
 		if respHeaders, err := str.Header(); err == nil {
 			handler.OnReceiveHeaders(respHeaders)
 		}
+	}
 
 	// Download each response message
 	for err == nil {
@@ -288,7 +290,9 @@ func invokeServerStream(ctx context.Context, stub grpcdynamic.Stub, md *desc.Met
 		return fmt.Errorf("grpc call for %q failed: %v", md.GetFullyQualifiedName(), err)
 	}
 
+	if str != nil {
 		handler.OnReceiveTrailers(stat, str.Trailer())
+	}
 
 	return nil
 }