Check that http_request.query.force_ssjs_theme is defined before using

the first element.

web/lib/html_themes.ssjs

@@ -12,5 +12,9 @@ if(file_exists(system.data_dir+'user/'+format("%04d.html_theme",user.number)))
 if(Themes[CurrTheme] == undefined || Themes[CurrTheme].dir == undefined)
 	CurrTheme=DefaultTheme;
 
-if(http_request.query.force_ssjs_theme[0] != undefined && Themes[http_request.query.force_ssjs_theme[0]] != undefined)
-	CurrTheme=http_request.query.force_ssjs_theme[0];
+if(http_request.query.force_ssjs_theme != undefined) {
+	if(http_request.query.force_ssjs_theme[0] != undefined && Themes[http_request.query.force_ssjs_theme[0]] != undefined)
+		CurrTheme=http_request.query.force_ssjs_theme[0];
+}
+
+writeln("Theme: "+CurrTheme);