The plugin generated 2 characters of unexpected output during activation. If you

When I try to activate my plugin it shows me this warning, although in version 3.4 it worked fine, without warnings while activating.

My activation function code:

<?php
	/*
	* Plugin Name: Google Maps Plugin
	* Description: Google Maps
	* Version: 1.0
	*/
	if (!isset($wpdb)) $wpdb = $GLOBALS['wpdb'];
	$wpdb->gm_maps_table=$wpdb->prefix . "gm_maps";
	$wpdb->gm_marker_table=$wpdb->prefix . "gm_marker_table";
	$wpdb->gm_driving_table=$wpdb->prefix . "gm_directions_table";
	$wpdb->gm_styled_maps_table=$wpdb->prefix . "gm_styled_maps";

	require_once(plugin_dir_path(__FILE__) . 'extract_images.php');
	require_once(plugin_dir_path(__FILE__) . 'driving.php');
	require_once(plugin_dir_path(__FILE__) . 'styled_maps.php');

	function gm_activate(){
		global $wpdb;
		require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
		//create maps table
		$query="CREATE TABLE IF NOT EXISTS {$wpdb->gm_maps_table}(id int(100) NOT NULL AUTO_INCREMENT,
		map_title varchar(255) DEFAULT 'Map',
		map_width int(11) DEFAULT 400,
		map_height int(11) DEFAULT 400,
		map_start_lat varchar(20) DEFAULT NULL,
		map_start_lng varchar(20) DEFAULT NULL,
		styled_map tinyint(1) DEFAULT 1,
		map_type varchar(20) DEFAULT 'ROADMAP',
		start_zoom int(2) DEFAULT 8,
		UNIQUE KEY  id (id));";
		dbDelta($query);
		//first map if table is empty
		$check=$wpdb->get_results("SELECT * FROM {$wpdb->gm_maps_table}");
		if(!$check){
			$wpdb->insert($wpdb->gm_maps_table, array('id'=>1,
					'map_title'=>"First map",
					'map_height'=>300,
					'map_width'=>300,
					'map_start_lat'=>"44.5435289",
					'map_start_lng'=>"26.8569689",
					'styled_map'=>0,
					'map_type'=>"ROADMAP"));
		}

		//create marker table
		$query="CREATE TABLE IF NOT EXISTS {$wpdb->gm_marker_table}(id int(100) NOT NULL AUTO_INCREMENT,
		marker_title varchar(255) NOT NULL DEFAULT 'marker',
		map_id varchar(255),
		m_start_lat varchar(20) DEFAULT NULL,
		m_start_lng varchar(20) DEFAULT NULL,
		text text(10000),
		icon_type varchar(255) DEFAULT 'gm_default',
		icon varchar(255) DEFAULT 'gm_default_marker.png',
		UNIQUE KEY  id (id));";
		dbDelta($query);
		//first marker if marker table is empty
		$check=$wpdb->get_results("SELECT * FROM {$wpdb->gm_marker_table}");
		if(!$check){
			$wpdb->insert($wpdb->gm_marker_table, array('map_id'=>1,
					'm_start_lat'=>44,
					'm_start_lng'=>26,
					'text'=>"first marker",
					'icon_type'=>"default",
					'icon'=>'gm_default_marker.png'));
		}

		//create driving table
		$query="CREATE TABLE IF NOT EXISTS {$wpdb->gm_driving_table}(
		id int(100) NOT NULL AUTO_INCREMENT,
		map_name varchar(255) DEFAULT 'Directions map',
		height int(4) DEFAULT 300,
		width int(4) DEFAULT 300,
		map_type varchar(255) DEFAULT 'ROADMAP',
		map_center varchar(255) NOT NULL,
		zoom int(3) DEFAULT 8,
		value text(1000),
		dir_panel int(1) DEFAULT 1,
		travel_mode varchar(255) DEFAULT 'DRIVING',
		UNIQUE KEY  id (id));";
		dbDelta($query);
		$check=$wpdb->get_results("SELECT * FROM {$wpdb->gm_driving_table}");
		if(!$check){
			$wpdb->insert($wpdb->gm_driving_table, array('id'=>1,
					'map_name'=>'First directions map',
					'height'=>600,
					'width'=>600,
					'map_center'=>'40.66110427536902,-73.936015',
					'value'=>'{\"start\":{\"lat\":40.70473,\"lng\":-73.90690000000001},\"end\":{\"lat\":40.61745000000001,\"lng\":-73.96512999999999},\"waypoints\":[{\"location\":{\"lat\":40.6766196,\"lng\":-73.96393560000001},\"stopover\":false}]}',
					'travel_mode'=>'DRIVING',
					'dir_panel'=>1,
					'zoom'=>12));
		}

		//create styled maps table
		$query="CREATE TABLE IF NOT EXISTS {$wpdb->gm_styled_maps_table}(id int(100) NOT NULL AUTO_INCREMENT,
		name varchar(255) NOT NULL DEFAULT 'Styled map',
		value text(10000),
		UNIQUE KEY  id (id));";
		dbDelta($query);

		$str=json_encode(array(array("featureType"=>'road', "elementType"=>'geometry', "stylers"=>array(array("visibility"=>'on', "color"=>'#808080', "hue"=>'#00ffff')))));
		$check=$wpdb->get_results("SELECT * FROM {$wpdb->gm_styled_maps_table}");
		if(!$check){
			$wpdb->insert($wpdb->gm_styled_maps_table, array('id'=>1,
					'name'=>"First styled map",
					'value'=>$str));
		}
	}
	register_activation_hook(__FILE__, 'gm_activate');
?>

I’ve checked on white spaces before and after <?php ?>.
Anyone can help me to fix this problem?