fix: crash when trying to checkout a remote branch with a local branch has no upstream (#2664)

Author: love-linger

src/ViewModels/Pull.cs

@@ -205,7 +205,7 @@ private void PostRemoteSelected()
             {
                 foreach (var branch in branches)
                 {
-                    if (Current.Upstream == branch.FullName)
+                    if (Current.Upstream.Equals(branch.FullName, System.StringComparison.Ordinal))
                     {
                         SelectedBranch = branch;
                         autoSelectedBranch = true;

src/ViewModels/Push.cs

@@ -136,23 +136,23 @@ public Push(Repository repo, Models.Branch localBranch)
             }
 
             // Find preferred remote if selected local branch has upstream.
-            if (!string.IsNullOrEmpty(_selectedLocalBranch?.Upstream) && !_selectedLocalBranch.IsUpstreamGone)
+            if (_selectedLocalBranch != null)
             {
-                _tracking = false;
-
-                foreach (var branch in repo.Branches)
+                var upstream = _selectedLocalBranch.Upstream;
+                if (!string.IsNullOrEmpty(upstream) && !_selectedLocalBranch.IsUpstreamGone)
                 {
-                    if (!branch.IsLocal && _selectedLocalBranch.Upstream == branch.FullName)
+                    _tracking = false;
+
+                    foreach (var branch in repo.Branches)
                     {
-                        _selectedRemote = repo.Remotes.Find(x => x.Name == branch.Remote);
-                        break;
+                        if (!branch.IsLocal && upstream.Equals(branch.FullName, StringComparison.Ordinal))
+                        {
+                            _selectedRemote = repo.Remotes.Find(x => x.Name == branch.Remote);
+                            break;
+                        }
                     }
                 }
             }
-            else
-            {
-                _tracking = true;
-            }
 
             // Set default remote to the first if it has not been set.
             if (_selectedRemote == null)

src/ViewModels/Repository.cs

@@ -1451,6 +1451,7 @@ public async Task CheckoutBranchAsync(Models.Branch branch)
                 foreach (var b in _branches)
                 {
                     if (b.IsLocal &&
+                        !string.IsNullOrEmpty(b.Upstream) &&
                         b.Upstream.Equals(branch.FullName, StringComparison.Ordinal) &&
                         b.Ahead.Count == 0)
                     {

src/Views/BranchTree.axaml.cs

@@ -704,7 +704,7 @@ private ContextMenu CreateContextMenuForLocalBranch(ViewModels.Repository repo,
         {
             var current = repo.CurrentBranch;
             var menu = new ContextMenu();
-            var upstream = repo.Branches.Find(x => x.FullName.Equals(branch.Upstream, StringComparison.Ordinal));
+            var upstream = !string.IsNullOrEmpty(branch.Upstream) ? repo.Branches.Find(x => x.FullName.Equals(branch.Upstream, StringComparison.Ordinal)) : null;
 
             var push = new MenuItem();
             push.Header = App.Text("BranchCM.Push", branch.Name);