fix: fix various nullable issue.

src/Ursa/Controls/DateTimePicker/DateTimePicker.cs

@@ -140,7 +140,7 @@ public class DateTimePicker : DatePickerBase
         }
     }
 
-    private void OnTimeSelectedChanged(object sender, TimeChangedEventArgs e)
+    private void OnTimeSelectedChanged(object? sender, TimeChangedEventArgs e)
     {
         if (SelectedDate is null)
         {

src/Ursa/Controls/DateTimePicker/TimePicker.cs

@@ -93,7 +93,7 @@ public class TimePicker : TimePickerBase, IClearControl
         SyncTimeToText(SelectedTime);
     }
 
-    private void OnPresenterTimeChanged(object sender, TimeChangedEventArgs e)
+    private void OnPresenterTimeChanged(object? sender, TimeChangedEventArgs e)
     {
         if (_suppressTextPresenterEvent) return;
         SetCurrentValue(SelectedTimeProperty, e.NewTime);

src/Ursa/Controls/DateTimePicker/TimeRangePicker.cs

@@ -156,7 +156,7 @@ public class TimeRangePicker : TimePickerBase, IClearControl
         SyncTimeToText(EndTime, false);
     }
 
-    private void OnPresenterTimeChanged(object sender, TimeChangedEventArgs e)
+    private void OnPresenterTimeChanged(object? sender, TimeChangedEventArgs e)
     {
         if (_suppressTextPresenterEvent) return;
         SetCurrentValue(Equals(sender, _startPresenter) ? StartTimeProperty : EndTimeProperty, e.NewTime);

tests/HeadlessTest.Ursa/Controls/FormTests/Dynamic_Item_Generation/Test.cs

@@ -26,6 +26,7 @@ public class Test
         window.Show();
 
         var form = window.FindControl<Form>("Form");
+        Assert.NotNull(form);
         var logicalChildren = form.GetLogicalChildren().ToList();
         Assert.True(logicalChildren.All(a=>a is FormItem));
         Assert.IsType<TextBox>(logicalChildren[0].LogicalChildren[0]);
@@ -65,6 +66,7 @@ public class Test
         window.Show();
 
         var form = window.FindControl<Form>("Form");
+        Assert.NotNull(form);
         var logicalChildren = form.GetLogicalChildren().ToList();
         Assert.True(logicalChildren.All(a=>a is FormGroup));
         var formGroup = (FormGroup)logicalChildren[0];
@@ -108,6 +110,7 @@ public class Test
         window.Show();
 
         var form = window.FindControl<Form>("Form");
+        Assert.NotNull(form);
         var logicalChildren = form.GetLogicalChildren().ToList();
         Assert.True(logicalChildren.All(a=>a is FormItem || a is FormGroup));
         Assert.IsType<TextBox>(logicalChildren[0].LogicalChildren[0]);