b/325757513 Ignore double-clicks when no file selected (#1304)

This fixes a spuriuous source of NullReferenceExceptions.

Author: jpassing

sources/Google.Solutions.Mvvm/Controls/FileBrowser.cs

@@ -288,7 +288,8 @@ private async void fileList_DoubleClick(object sender, EventArgs args)
                 throw new InvalidOperationException("Control is not bound");
             }
 
-            if (this.fileList.SelectedModelItem.Type.IsFile)
+            if (this.fileList.SelectedModelItem == null ||
+                this.fileList.SelectedModelItem.Type.IsFile)
             {
                 return;
             }