make sure _jsonElement is filled with a default value so that Old and…

… New do not throw

src/HassModel/NetDaemon.HassModel.Tests/Entities/EntityTest.cs

@@ -102,4 +102,18 @@ public void ShouldWrapAreaFromContext()
         // Assert
         Assert.Equal("Area Name", target.Area);
     }
+
+    [Fact]
+    public void StateChange_WithTestConstructor_PropertiesShouldNotThrow()
+    {
+        // Arrange
+        var haContextMock = new Mock<IHaContext>();
+        var target = new TestEntity(haContextMock.Object, "domain.testEntity");
+        var stateChange = new StateChange(target, null, null);
+
+        // Act & Assert
+        Assert.NotNull(stateChange.Entity);
+        Assert.Null(stateChange.Old);
+        Assert.Null(stateChange.New);
+    }
 }

src/HassModel/NetDeamon.HassModel/Entities/StateChange.cs

@@ -31,13 +31,15 @@ internal StateChange(JsonElement jsonElement, IHaContext haContext)
     public StateChange(Entity entity, EntityState? old, EntityState? @new)
     {
         _entity = entity;
-        _new    = @new;
-        _old    = old;
+        _new = @new;
+        _old = old;
         _haContext = null!; // haContext is not used when _entity is already initialized
+        _jsonElement = JsonSerializer.SerializeToDocument(new HassStateChangedEventData()).RootElement; // Use a default value so _jsonElement.GetProperty is not going to throw.
     }
 
     /// <summary>The Entity that changed</summary>
-    public virtual Entity Entity => _entity ??= new Entity(_haContext, _jsonElement.GetProperty("entity_id").GetString() ?? throw  new InvalidOperationException("No Entity_id in state_change event"));
+    public virtual Entity Entity =>
+        _entity ??= new Entity(_haContext, _jsonElement.GetProperty("entity_id").GetString() ?? throw new InvalidOperationException("No Entity_id in state_change event"));
 
     /// <summary>The old state of the entity</summary>
     public virtual EntityState? Old => _old ??= _jsonElement.GetProperty("old_state").Deserialize<EntityState>();