Book allows emptying node titles in admin/content/book/* (Notice: Trying to get property of non-object in node_page_title)

After emptying a node title in admin/content/book/* your site gives errrors of course and the title can only be fixed in this same book form, not through the node editing form...
Why on earth are these field not required?

I have not investigated this, just reporting it now.

D8.4.x

Still reproducible.
Git 8.4.x 1484dbf430ff38ef8eb419c4297355ef800fdbc9 2017-08-13 16:34:12 +0900

On the /admin/structure/book/{book-id} page the node titles can be edited, and when one of the textfield is blank then I get the following error message:

The website encountered an unexpected error. Please try again later.</br></br><em class="placeholder">Drupal\Core\Entity\EntityStorageException</em>: SQLSTATE[23000]: Integrity constraint violation: 1048 Column &#039;title&#039; cannot be null: INSERT INTO {node_field_data} (nid, vid, type, langcode, status, title, uid, created, changed, promote, sticky, revision_translation_affected, default_langcode) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10, :db_insert_placeholder_11, :db_insert_placeholder_12); Array
(
    [:db_insert_placeholder_0] =&gt; 12
    [:db_insert_placeholder_1] =&gt; 53
    [:db_insert_placeholder_2] =&gt; book
    [:db_insert_placeholder_3] =&gt; en
    [:db_insert_placeholder_4] =&gt; 1
    [:db_insert_placeholder_5] =&gt;
    [:db_insert_placeholder_6] =&gt; 0
    [:db_insert_placeholder_7] =&gt; 1502214772
    [:db_insert_placeholder_8] =&gt; 1502619299
    [:db_insert_placeholder_9] =&gt; 0
    [:db_insert_placeholder_10] =&gt; 0
    [:db_insert_placeholder_11] =&gt; 1
    [:db_insert_placeholder_12] =&gt; 1
)