Linq查询导致访问链接表时出现空引用异常

var result = dbContext.ParentTable.Where(p => p.ChildTable != null && p.ChildTable.Property == value).ToList();

var result = dbContext.ParentTable.Where(p => p.ChildTable.Property == value).ToList();

var result = dbContext.ParentTable.Join(dbContext.ChildTable,
    parent => parent.Id,
    child => child.ParentId,
    (parent, child) => new { Parent = parent, Child = child })
    .Where(pc => pc.Child.Property == value)
    .Select(pc => pc.Parent)
    .ToList();