Fatal database error when updating to 2.0-beta9: "Invalid use of NULL value"

Thanks for filing the ticket and for the fix!
Committed and tagged beta10 with this fix, since it's better to avoid beta9 at this point.
https://www.drupal.org/project/entity_usage/releases/8.x-2.0-beta10 →

Thanks for the quick response!

Automatically closed - issue fixed for 2 weeks with no activity.

On entity_usage_update_8206() I got:

>  [notice] Update started: entity_usage_update_8206                                                                                                                                                   
>  [error]  SQLSTATE[HY000]: General error: 3750 Unable to create or change a table without a primary key, when the system variable 'sql_require_primary_key' is set. Add a primary key to the table or unset this variable to avoid this message. Note that tables without a primary key can cause performance problems in row-based replication, so please consult your DBA before changing this setting  .: ALTER TABLE `entity_usage` DROP PRIMARY KEY;

I have to disable 'sql_require_primary_key' on session level by adding code to entity_usage_update_8206():$database->query('SET SESSION sql_require_primary_key=0;');
before calling:

// Drop all indices involving columns we want to change.

I got the error as descriped in attached file. It handles about the field "tagert_id_string" which doesn't have a default value.
After a lot of searching I found the error and the solution:
In MySQL under entity_usage I inspected the table.
The field "target_id_string" is created with no NULL-value and no default value.
The field can not have at NULL-value, but it MUST have at default value.
I set (in) 0 as the default value and that solved my problem.

From the patch:

+  $database->update('entity_usage')
+    ->fields(['target_id_string' => ''])
+    ->condition('target_id_string', NULL, 'IS NULL')
+    ->execute();

I think that line 3 ('->condition('target_id_string', NULL, 'IS NULL')') should be different?