fix: 得先在music.xml里有longmusic后才能用mcm改

Author: clansty

MaiChartManager/Models/MusicXml.cs

@@ -457,7 +457,13 @@ public bool LongMusic
         set
         {
             Modified = true;
-            RootNode.SelectSingleNode("longMusic").InnerText = value ? "1" : "0";
+            var node = RootNode.SelectSingleNode("longMusic");
+            if (node == null)
+            {
+                node = RootNode.OwnerDocument.CreateElement("longMusic");
+                RootNode.AppendChild(node);
+            }
+            node.InnerText = value ? "1" : "0";
         }
     }