Debugger: Fix crash in memory agent

Change-Id: I553858650102a93a90ebeae7ce3b5a29e973ace7
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/memoryagent.cpp

@@ -284,12 +284,10 @@ MemoryAgent::MemoryAgent(const MemoryViewSetupData &data, DebuggerEngine *engine
 
 MemoryAgent::~MemoryAgent()
 {
-    if (m_service) {
-        if (m_service->editor())
-            EditorManager::closeDocument(m_service->editor()->document());
-        if (m_service->widget())
-            m_service->widget()->close();
-    }
+    if (m_service && m_service->editor())
+        EditorManager::closeDocument(m_service->editor()->document());
+    if (m_service && m_service->widget()) // m_service might be set to null by closeDocument
+        m_service->widget()->close();
 }
 
 void MemoryAgent::updateContents()