Solved Calling function of other plugin

Hey, I try to call functions of another plugin.
MAINPLUGIN:

Code:

public class EconomyManager extends JavaPlugin implements Listener {
   
    ServerManager serverManager;
   
    public void onEnable() {
        //urpsrünglich this
        serverManager = (ServerManager) getServer().getPluginManager().getPlugin("server_manager");
        serverManager.getServerManagerInstance().sendTest();
    }
}

And this MAINPLUGIN should call functions of this SECONDPLUGIN:

Code:

public class ServerManager extends JavaPlugin {
   
    public ServerManager serverManager;
   
    public ServerManager getServerManagerInstance() {
        return this.serverManager;
    }
   
    public void onEnable() {
        serverManager = this;
    }
   
    public void sendTest() {
        getServer().getConsoleSender().sendMessage(ChatColor.GREEN + "[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[");
    }
}

My current problem: It says

What can I do to fix this? Did I make a stupid mistake anywhere?
Thanks in advance, Marc

 

@timtower Hey, I can not see a link if you sent one, but this code I made with help of a few other posts of this forum. I am basically a Javascript and C# developer and here calling a function of the other plugin feels so complicated. The other thing is that I can not find a cool side with clear examples, for example how to call a function of a other plugins. So I am happy for every help.

 

@timtower Thanks for your reply.

1) There is much more code, but I cut off all the irrelevant functions, which are not important for this issue. This makes the code more clearer for everybody. For this, I only have to call the function sendTest().
2) Yes everything compiled correctly.
3) Yes, everything is on the same server.
4) Yes, there is only one Plugin called server_manager.

Thanks for your patience and help.

 

@timtower No, the error happens in the console. I already added the plugin to the IDE Project library. BUT. I am stupid. You were right. There was much other code. And in this code, there was the error. I am so sorry, but I admire and thank you for your help.
Thank you.