大家好 看看这是什么错，遇到这种情况如何调试 flex有什么比较好的调试工具

TypeError: Error #1009: 无法访问空对象引用的属性或方法。
at com.esria.samples.networkMonitor.view::DeviceLayout/addThumbs()[D:\Program Files\MyEclipse 6.0\workspace\NetWorkMonitor\src\com\esria\samples\networkMonitor\view\DeviceLayout.mxml:156]
at com.esria.samples.networkMonitor.view::DeviceLayout/updateView()[D:\Program Files\MyEclipse 6.0\workspace\NetWorkMonitor\src\com\esria\samples\networkMonitor\view\DeviceLayout.mxml:106]
at com.esria.samples.networkMonitor.view::DeviceLayout/set items()[D:\Program Files\MyEclipse 6.0\workspace\NetWorkMonitor\src\com\esria\samples\networkMonitor\view\DeviceLayout.mxml:308]
at <anonymous>()[E:\dev\3.0.x\frameworks\projects\framework\src\mx\binding\utils\BindingUtils.as:102]
at mx.binding.utils::ChangeWatcher/wrapHandler()[E:\dev\3.0.x\frameworks\projects\framework\src\mx\binding\utils\ChangeWatcher.as:427]
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at com.esria.samples.networkMonitor.model::Application/sort()[D:\Program Files\MyEclipse 6.0\workspace\NetWorkMonitor\src\com\esria\samples\networkMonitor\model\Application.as:76]
at com.esria.samples.networkMonitor.model::Application/set devices()[D:\Program Files\MyEclipse 6.0\workspace\NetWorkMonitor\src\com\esria\samples\networkMonitor\model\Application.as:45]
at com.esria.samples.networkMonitor.command::GetDevicesCommand/result()[D:\Program Files\MyEclipse 6.0\workspace\NetWorkMonitor\src\com\esria\samples\networkMonitor\command\GetDevicesCommand.as:27]
at mx.rpc::AsyncToken/http://www.adobe.com/2006/flex/mx/internal::applyResult()[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\rpc\AsyncToken.as:197]
at mx.rpc.events::ResultEvent/http://www.adobe.com/2006/flex/mx/internal::callTokenResponders()[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\rpc\events\ResultEvent.as:159]
at mx.rpc::AbstractOperation/http://www.adobe.com/2006/flex/mx/internal::dispatchRpcEvent()[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\rpc\AbstractOperation.as:184]
at mx.rpc::AbstractInvoker/http://www.adobe.com/2006/flex/mx/internal::resultHandler()[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\rpc\AbstractInvoker.as:191]
at mx.rpc::Responder/result()[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\rpc\Responder.as:41]
at mx.rpc::AsyncRequest/acknowledge()[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\rpc\AsyncRequest.as:74]
at NetConnectionMessageResponder/resultHandler()[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\messaging\channels\NetConnectionChannel.as:469]
at mx.messaging::MessageResponder/result()[E:\dev\3.0.x\frameworks\projects\rpc\src\mx\messaging\MessageResponder.as:199]