fix errors

Author: KikaDeveloper

ViewModels/ProjectViewModel.cs

@@ -84,6 +84,7 @@ private async System.Threading.Tasks.Task AddNewTask()
             if(newTask != null)
             {
                 newTask.SubTasks = new ObservableCollection<SubTask>();
+                Project.Tasks!.Add(newTask);
 
                 var taskVM = new TaskViewModel(newTask);
                 taskVM.Task.ProjectId = Project.Id;

ViewModels/TaskViewModel.cs

@@ -112,6 +112,7 @@ private async System.Threading.Tasks.Task AddSubTask()
 
             if(newTask != null)
             {
+                Task.SubTasks!.Add(newTask);
                 var newSubTaskVM = new SubTaskViewModel(newTask);
                 newSubTaskVM.Task.TaskId = Task.Id;
                 newSubTaskVM.SubTaskDeleteEvent += DeleteSubTaskEventHandler;