nf-ajax-mu-plugin.php

<?php

function nf_test_exclude_plugins( $plugins ) {
	if ( !defined( 'DOING_AJAX' ) || !DOING_AJAX || !isset( $_REQUEST['action'] ) || ( 'nf_output_field_settings_html' != $_REQUEST['action'] && 'ninja_forms_new_field' != $_REQUEST['action'] ) ) return $plugins;

	foreach( $plugins as $key => $plugin ) {
		if ( false !== strpos( $plugin, 'ninja-forms' ) ) continue;
		unset( $plugins[$key] );
	}

	return $plugins;
}
add_filter( 'option_active_plugins', 'nf_test_exclude_plugins' );