WordPress plugin admin page loads on every admin page, above doctype declaration
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I've written a Wordpress plugin consisting of two files. One sets the parameters for the plugin to function, the second file is the actual function. Everything works fine except one thing:
In WP administrator, the plugin admin page is loaded above the doctype declaration, and on every page. My guess is that it has to do with the parameter file. Here's the code:

    <?php
    define( 'PLUGIN_DIR', dirname(__FILE__).'/' ); 

    add_action( 'admin_menu' , 'verkoopstatus_admin_menu' );

    function verkoopstatus_admin_menu() {
   	add_menu_page('Beheer verkoopstatus', 'Verkoopstatus', 'manage_options', 'Verkoopstatus', 'showHome', plugins_url('verkoopstatus/images/verkoopstatus-icon-2.png') );
	if ( is_admin() ) { //run only in WP admin area
	}
	require( plugin_dir_path( __FILE__ ) . 'verkoopstatus.php');
    }
    ?>

Does anyone have a clue what I'm doing wrong? I've tried various changes, but can't find the correct method. In the second file (verkoopstatus.php), I put all HTML/CSS code within the wrap class.