feat: don't panic when provider not found in Login() API (#3659)

controllers/auth.go

@@ -598,6 +598,9 @@ func (c *ApiController) Login() {
 			c.ResponseError(err.Error())
 			return
 		}
+		if provider == nil {
+			c.ResponseError(fmt.Sprintf(c.T("auth:The provider: %s does not exist"), authForm.Provider))
+		}
 
 		providerItem := application.GetProviderItem(provider.Name)
 		if !providerItem.IsProviderVisible() {