fix empty/missing logouturl leads to error, add description to config.yml.sample

config.yml.sample

@@ -311,6 +311,9 @@ auth:
       - name:
         # The auth url to send users to if they want to authenticate using OpenID Connect.
         authurl:
+        # The oidc logouturl that users will be redirected to on logout
+        # leave empty or delete key, if you do not want to be redirected
+        logouturl:
         # The client ID used to authenticate Vikunja at the OpenID Connect provider.
         clientid:
         # The client secret used to authenticate Vikunja at the OpenID Connect provider.

pkg/modules/auth/openid/providers.go

@@ -120,13 +120,18 @@ func getProviderFromMap(pi map[string]interface{}) (provider *Provider, err erro
 
 	k := getKeyFromName(name)
 
+	logoutUrl, ok := pi["logout_url"].(string)
+	if !ok {
+		logoutUrl = ""
+	}
+
 	provider = &Provider{
 		Name:            pi["name"].(string),
 		Key:             k,
 		AuthURL:         pi["authurl"].(string),
 		OriginalAuthURL: pi["authurl"].(string),
 		ClientSecret:    pi["clientsecret"].(string),
-		LogoutURL:       pi["logouturl"].(string),
+		LogoutURL:       logoutUrl,
 	}
 
 	cl, is := pi["clientid"].(int)