fix: pulling with empty repository (no local branch) crashes this app (#1020)

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

Author: love-linger

src/ViewModels/Repository.cs

@@ -663,6 +663,12 @@ public void Pull(bool autoStart)
                 return;
             }
 
+            if (_currentBranch == null)
+            {
+                App.RaiseException(_fullpath, "Can NOT found current branch!!!");
+                return;
+            }
+
             var pull = new Pull(this, null);
             if (autoStart && pull.SelectedBranch != null)
                 ShowAndStartPopup(pull);