Avoid panic if RG or ASE do not exist when creating an App Service (h…

…ashicorp#12518)

Since the `Get` function of the `AppServices` client does not return an
error if the resource is not found, the code behaves as if everything is
fine and attempts to access fields of a struct that have not been
un-marshalled, since the API responded with 404.

azurerm/internal/services/web/app_service_resource.go

@@ -236,8 +236,9 @@ func resourceAppServiceCreate(d *pluginsdk.ResourceData, meta interface{}) error
 	// Check if App Service Plan is part of ASE
 	// If so, the name needs updating to <app name>.<ASE name>.appserviceenvironment.net and FQDN setting true for name availability check
 	aspDetails, err := aspClient.Get(ctx, aspID.ResourceGroup, aspID.ServerfarmName)
-	if err != nil {
-		return fmt.Errorf("App Service Environment %q (Resource Group %q) does not exist", aspID.ServerfarmName, aspID.ResourceGroup)
+	// 404 is incorrectly being considered an acceptable response, issue tracked at https://github.com/Azure/azure-sdk-for-go/issues/15002
+	if err != nil || utils.ResponseWasNotFound(aspDetails.Response) {
+		return fmt.Errorf("App Service Environment %q or Resource Group %q does not exist", aspID.ServerfarmName, aspID.ResourceGroup)
 	}
 	if aspDetails.HostingEnvironmentProfile != nil {
 		availabilityRequest.Name = utils.String(fmt.Sprintf("%s.%s.appserviceenvironment.net", name, *aspDetails.HostingEnvironmentProfile.Name))