ux: validation tips for patch file in Apply popup

Signed-off-by: leo <longshuang@msn.cn>

Author: love-linger

src/ViewModels/Apply.cs

@@ -31,11 +31,7 @@ public Models.ApplyWhiteSpaceMode SelectedWhiteSpaceMode
         public bool FromClipboard
         {
             get => _fromClipboard;
-            set
-            {
-                if (SetProperty(ref _fromClipboard, value))
-                    ValidateProperty(_patchFile, nameof(PatchFile));
-            }
+            set => SetProperty(ref _fromClipboard, value);
         }
 
         public bool ThreeWayMerge
@@ -61,10 +57,16 @@ public static ValidationResult ValidatePatchFile(string file, ValidationContext
             if (ctx.ObjectInstance is not Apply apply)
                 return new ValidationResult("Invalid object instance!!!");
 
-            if (apply.FromClipboard || File.Exists(file))
+            if (apply.FromClipboard)
                 return ValidationResult.Success;
 
-            return new ValidationResult($"File '{file}' can NOT be found!!!");
+            if (string.IsNullOrEmpty(file))
+                return new ValidationResult("Please select a patch file!!!");
+
+            if (!File.Exists(file))
+                return new ValidationResult($"File '{file}' can NOT be found!!!");
+
+            return ValidationResult.Success;
         }
 
         public override async Task<bool> Sure()