wp-theme-ninja-forms.php

<?php

$form_id = 1;

// Check which version of Ninja Forms.
if( ! get_option( 'ninja_forms_load_deprecated', FALSE ) ){

    // Ninja Forms THREE.

    // Check if Ninja Forms is loaded.
    if( function_exists( 'Ninja_Forms' ) ){

        // Output form by ID.
        Ninja_Forms()->display( $form_id );
    }
} else {

    // Ninja Forms v2.9.x (DEPRECATED).

    // Check if the Ninja Forms display function exists.
    if( function_exists( 'ninja_forms_display_form' ) ){

        // Output the form by ID.
        ninja_forms_display_form( $form_id );
    }
}