fix an npe (#17)

Author: devoncarew

lib/memory/memory.dart

@@ -313,7 +313,7 @@ class MemoryChart extends LineChart<MemoryTracker> {
     }
 
     // display the process usage
-    final String rss = '${_printMb(data.processRss, 0)} MB RSS';
+    final String rss = '${_printMb(data.processRss ?? 0, 0)} MB RSS';
     processLabel.text = rss;
 
     // display the dart heap usage

lib/tables.dart

@@ -13,8 +13,6 @@ import 'utils.dart';
 
 // TODO(devoncarew): fixed position header
 
-// TODO(devoncarew): virtualize
-
 class Table<T> extends Object with SetStateMixin {
   final CoreElement element;
   final bool _isVirtual;