package wicket.contrib.gmap3;

import org.apache.wicket.ResourceReference;
import org.apache.wicket.ajax.WicketAjaxReference;
import org.apache.wicket.behavior.HeaderContributor;
import org.apache.wicket.markup.html.IHeaderContributor;
import org.apache.wicket.markup.html.IHeaderResponse;
import org.apache.wicket.markup.html.WicketEventReference;
import org.apache.wicket.markup.html.resources.JavascriptResourceReference;

public class GMapHeaderContributor extends HeaderContributor {
    private static final long serialVersionUID = 1L;

    // URL for Google Maps' API endpoint.
    private static final String GMAP_API_URL = "://maps.google.com/maps/api/js?v=3&sensor=false";

    private static final String HTTP = "http";

    // We have some custom Javascript.
    private static final ResourceReference WICKET_GMAP_JS = new JavascriptResourceReference( GMap.class, "wicket-gmap.js" );

    protected static final String EMPTY = "";

    String _clientId;

    public GMapHeaderContributor() {
        this( HTTP, null );
    }

    public GMapHeaderContributor( final String schema, final String clientId ) {
        super( new IHeaderContributor() {
            private static final long serialVersionUID = 1L;

            @Override
            public void renderHead( IHeaderResponse response ) {
                final String clientParm;
                if ( !EMPTY.equals( clientId ) ) {
                    clientParm = "&client=" + clientId;
                } else {
                    clientParm = EMPTY;
                }
                response.renderJavascriptReference( schema + GMAP_API_URL + clientParm );
                response.renderJavascriptReference( WicketEventReference.INSTANCE );
                response.renderJavascriptReference( WicketAjaxReference.INSTANCE );
                response.renderJavascriptReference( WICKET_GMAP_JS );

            }
        } );
    }

    String getClientId() {
        return _clientId;
    }
}

package wicket.contrib.gmap3;

import org.apache.wicket.ResourceReference;
import org.apache.wicket.ajax.WicketAjaxReference;
import org.apache.wicket.behavior.HeaderContributor;
import org.apache.wicket.markup.html.IHeaderContributor;
import org.apache.wicket.markup.html.IHeaderResponse;
import org.apache.wicket.markup.html.WicketEventReference;
import org.apache.wicket.markup.html.resources.JavascriptResourceReference;

public class GMapHeaderContributor extends HeaderContributor {
    private static final long serialVersionUID = 1L;

    // URL for Google Maps' API endpoint.
    private static final String GMAP_API_URL = "://maps.google.com/maps/api/js?v=3&sensor=false";

    private static final String HTTP = "http";

    // We have some custom Javascript.
    private static final ResourceReference WICKET_GMAP_JS = new JavascriptResourceReference( GMap.class, "wicket-gmap.js" );

    protected static final String EMPTY = "";

    String _clientId;

    public GMapHeaderContributor() {
        this( HTTP, null );
    }

    public GMapHeaderContributor( final String schema, final String clientId ) {
        super( new IHeaderContributor() {
            private static final long serialVersionUID = 1L;

            @Override
            public void renderHead( IHeaderResponse response ) {
                final String clientParm;
                if ( clientId != null && !EMPTY.equals( clientId ) ) {
                    clientParm = "&client=" + clientId;
                } else {
                    clientParm = EMPTY;
                }
                response.renderJavascriptReference( schema + GMAP_API_URL + clientParm );
                response.renderJavascriptReference( WicketEventReference.INSTANCE );
                response.renderJavascriptReference( WicketAjaxReference.INSTANCE );
                response.renderJavascriptReference( WICKET_GMAP_JS );

            }
        } );
    }

    String getClientId() {
        return _clientId;
    }
}

fix nullcheck