Slim controller issue : must be an instance of ContainerInterface, instance of Slim\\Container given

Your code seems to be based on the Slim 3 documentation at http://www.slimframework.com/docs/objects/router.html which does not contain all the required code to avoid the Exception being thrown.

You basically have two options to make it work.

Option 1:

Import the namespace in your index.php, just like it is being done for Request and Response:

use \Interop\Container\ContainerInterface as ContainerInterface;

Option 2:

Change the constructor of the TopPageController to:

public function __construct(Interop\Container\ContainerInterface $ci) {
    $this->ci = $ci;
}

---

TL;DR

The reason the Exception is thrown is that the Slim\Container class is using the Interop\Container\ContainerInterface interface (see the source code):

use Interop\Container\ContainerInterface;

Since Slim\Container is extending Pimple\Container, the following should all be valid (working) type declarations for your controller's method:

public function __construct(Pimple\Container $ci) {
    $this->ci = $ci;
}

...or even...

public function __construct(ArrayAccess $ci) {
    $this->ci = $ci;
}

Based on a later change in Slim3 (from version 3.12.2 to 3.12.3) a slightly different ContainerInterface is required. This changes \Interop\ to \Psr\. Add the following on top of you code or change the existing use:

use Psr\Container\ContainerInterface;

Or change the constructor:

public function __construct(\Psr\Container\ContainerInterface $container)
{
    $this->container = $container;
}

Just paste the below code in your controller

use \Psr\Http\Message\ServerRequestInterface as Request;
use \Psr\Http\Message\ResponseInterface as Response;
use \Interop\Container\ContainerInterface as ContainerInterface;

Your controller's construct should look like the one below

public function __construct(ContainerInterface $container) {
    parent::__construct($container);
}

I think you are making mistake in giving namespace in controller for ContainerInterface.