Do not modify enumerated tree when plot is shown. (#853)

When going to sleep some node are removed on my PC and it crash with
InvalidOperationException because the collection is modified during
enumeration.

 So return a copy of the filtered list instead of yield return.

LibreHardwareMonitor/UI/TreeModel.cs

@@ -8,6 +8,7 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Linq;
 using Aga.Controls.Tree;
 
 namespace LibreHardwareMonitor.UI;
@@ -59,12 +60,10 @@ public class TreeModel : ITreeModel
         Node node = GetNode(treePath);
         if (node != null)
         {
-            foreach (Node n in node.Nodes)
-                if (_forceVisible || n.IsVisible)
-                    yield return n;
+            return node.Nodes.Where(n => _forceVisible || n.IsVisible).ToList();
         }
-        else
-        { }
+
+        return Enumerable.Empty<Node>();
     }
 
     public bool IsLeaf(TreePath treePath)
@@ -122,4 +121,4 @@ public class TreeModel : ITreeModel
         NodesRemoved?.Invoke(this, args);
     }
 
-}
+}