Apis.Get<...> returns null

You cannot use APIs as part of plugin configuration.  They are not always available in this stage of the lifecycle.  The earliest an API is available is on initialize.

It makes sense. Do you know best practice how to generate configuration later? I use INotifications and IDatabaseConnections there and for both often see exceptions.

What is it configuration for and why do you need to access these Apis?

Have to manage default notification settings for new users. So I need to get all notifications (names) from Apis and my current settings from DB.

This shouldn't be plugin configuration.  You should create a new page or panel to manage these settings.  Also note that unless the data you are getting from the database is from a custom table, its not supported.

I encourage you to look through the Developer training documents.  community.telligent.com/.../ui-extensibility

I use exists telligent table.

So you think, that there is no way to set configuration properties AFTER all apis initialize, am I right?

You can't use any existing core community tables.   It is not supported.   And no you can't do what you want to do as a plugin.  It has to be done as normal pages or panels.  But again, you cannot interact with the database directly at all, only your own tables.

As I see in the logs get ConfigurationOptions executes many times after plugin initialized. So I have isolated it from exceptions and now it seems to work as expected.

Catching an exception does not solve the problem.  It still is not going to work correctly, you are just now swallowing the actual issue.

As I said,  you need to create a page and widget to manage these settings or a custom management panel.   Plugin configuration will not work

I solved that issue in another way:

I created 1 settings field and store there a json object with all my notification settings. Then after server restart (plugin initialize) I update default notification table with stored data (do it on UserEvent_AfterCreate)

Everything works :)

I solved that issue in another way:

I created 1 settings field and store there a json object with all my notification settings. Then after server restart (plugin initialize) I update default notification table with stored data (do it on UserEvent_AfterCreate)

Everything works :)

What do you mean by default notification table, what table are you meaning?

That does not matter. It related to my task. Anyway, I found how I can store dynamic extension data. 

pieces of code:

public PropertyGroup[] ConfigurationOptions {
            get {
                var group = new PropertyGroup("options", "Options", 0);

                Property stored = new Property("stored", "stored", PropertyType.String, 0, "");
                stored.Visible = false;
                group.Properties.Add(stored);

                try {
                    if (props == null || props.Count == 0) prepareProps();

                    int counter = 0;
                    foreach (var item in props.OrderBy(i => i.Key)) {
                        item.Value.OrderNumber = counter++;
                        group.Properties.Add(item.Value);
                    }
                } catch (Exception e) {
                    log.error("", e);
                }

                return new[] {group};
            }
        }



Dictionary<string, bool> values = new Dictionary<string, bool>();

if (_configuration != null && _configuration.GetString("stored").Length > 10) {
                    string stored = _configuration.GetString("stored");
                    values = JsonConvert.DeserializeObject<Dictionary<string, bool>>(stored);
                    log.info("configuration restored");
                }




May be useful for example for data like list of groups or users or other objects with dynamic count. 

What are you trying to store here?  Plugin configuration is meant for just that, configuring the plugin, its not meant for extended or dynamic data storage like a table would be.  If you truly need to store custom data, you need to create a custom data store like a table.