Resolves #168 Fix error when entity view closed before history loaded

lib/entity_widgets/history_chart/entity_history.dart

@@ -32,6 +32,7 @@ class _EntityHistoryWidgetState extends State<EntityHistoryWidget> {
   List _history;
   bool _needToUpdateHistory;
   DateTime _historyLastUpdated;
+  bool _disposed = false;
 
   @override
   void initState() {
@@ -47,16 +48,20 @@ class _EntityHistoryWidgetState extends State<EntityHistoryWidget> {
     if (_historyLastUpdated == null || now.difference(_historyLastUpdated).inSeconds > 30) {
       _historyLastUpdated = now;
       ha.getHistory(entityId).then((history){
+        if (!_disposed) {
           setState(() {
             _history = history.isNotEmpty ? history[0] : [];
             _needToUpdateHistory = false;
           });
+        }
       }).catchError((e) {
         Logger.e("Error loading $entityId history: $e");
+        if (!_disposed) {
           setState(() {
             _history = [];
             _needToUpdateHistory = false;
           });
+        }
       });
     }
   }
@@ -131,4 +136,10 @@ class _EntityHistoryWidgetState extends State<EntityHistoryWidget> {
 
   }
 
+  @override
+  void dispose() {
+    _disposed = true;
+    super.dispose();
+  }
+
 }