Speculative fix for crash with buffer viewer being closed early.

* I received a few crash reports with a disposed buffer viewer still
  doing work. My theory is that some operation took long enough that the
  user was able to close the viewer (perhaps *because* it was taking a
  long time) and subsequent work then didn't handle the window having
  been closed.

renderdocui/Windows/BufferViewer.cs

@@ -1553,6 +1553,11 @@ namespace renderdocui.Windows
         {
             var state = GetUIState(type);
 
+            var bufView = state.m_GridView;
+
+            if(bufView.IsDisposed)
+                return;
+
             // only do this once, VSIn is guaranteed to be set (even if it's empty data)
             if(type == MeshDataStage.VSIn)
                 CalcCellFloatWidth();
@@ -1562,8 +1567,6 @@ namespace renderdocui.Windows
             if(data.Buffers == null)
                 return;
 
-            var bufView = state.m_GridView;
-
             bufView.RowCount = 0;
             state.m_Data = data;