refactor: use Models.Null instead of null to remove warings in Rider

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

Author: love-linger

src/ViewModels/Histories.cs

@@ -437,7 +437,7 @@ private void PostSelectedCommitsChanged()
             if (_selectedCommits.Count == 0)
             {
                 _repo.SearchCommitContext.Selected = null;
-                DetailContext = null;
+                DetailContext = new Models.Null();
             }
             else if (_selectedCommits.Count == 1)
             {
@@ -473,7 +473,7 @@ private void PostSelectedCommitsChanged()
         private Models.CommitGraph _graph = null;
         private List<Models.Commit> _selectedCommits = [];
         private Models.Bisect _bisect = null;
-        private object _detailContext = null;
+        private object _detailContext = new Models.Null();
         private bool _ignoreSelectionChange = false;
 
         private GridLength _leftArea = new GridLength(1, GridUnitType.Star);

src/Views/Histories.axaml

@@ -263,12 +263,11 @@
                   Focusable="False"
                   IsEnabled="{Binding #ThisControl.IsDetailsPanelExpanded, Mode=OneWay}"/>
 
-    <Grid Grid.Row="2" Grid.Column="0" Grid.ColumnSpan="3">
-      <ContentControl>
-        <ContentControl.Content>
-          <Binding Path="DetailContext">
-            <Binding.TargetNullValue>
-              <TabControl Padding="0,4">
+    <Grid Grid.Row="2" Grid.Column="0" Grid.ColumnSpan="3" >
+      <ContentControl Content="{Binding DetailContext, Mode=OneWay}">
+        <ContentControl.DataTemplates>
+          <DataTemplate DataType="m:Null">
+            <TabControl Padding="0,4">
                 <TabItem>
                   <TabItem.Header>
                     <Border Background="Transparent" PointerPressed="OnTabHeaderPointerPressed">
@@ -303,11 +302,8 @@
                   </TabItem.Header>
                 </TabItem>
               </TabControl>
-            </Binding.TargetNullValue>
-          </Binding>
-        </ContentControl.Content>
-        
-        <ContentControl.DataTemplates>
+          </DataTemplate>
+          
           <DataTemplate DataType="vm:CommitDetail">
             <v:CommitDetail IsDetailsPanelExpanded="{Binding #ThisControl.IsDetailsPanelExpanded, Mode=OneWay}"/>
           </DataTemplate>