Notice: Trying to access array offset on value of type null

Do you have a test case for this? I would like to be able to force the error to happen and then ensure that your patch fixes the problem without side effects. The way I read the code, $table is always a string, never an array. Indexing the string with [0] will grab the first character from the string to see if it is a ‘#’. Somehow, some function is passing NULL in as the table name (according to the backtrace you supplied). If anything, adding

if (empty($table)){
  return false;
}

is probably the way to fix this because I think your is_array() will always evaluate false, and the existance of temporary tables will always be false.

Your patch causes a failure in an existing Kernel Test

/**
   * Test the temporary table functionality.
   */
  public function testTemporaryTables() {

    $query = $this->connection->select('test_task', 't');
    $query->fields('t');

    $table = $this->connection->queryTemporary((string) $query);

    // First assert that the table exists.
    $this->assertTRUE(db_table_exists($table), 'The temporary table exists.');
}

I merged in my fix. Thanks for bringing this to my attention.

Here are a few core patches that have been rolled out to Drupal 9, but are still buggy in 8.8:

• #2867788-79: Log::findCaller fails to report the correct caller function with non-core drivers.
• #3128761-2: Duplicate timestamp placeholder in statistics query
• #3130655-3: Deprecate creating an instance of the class Drupal\Core\Database\Query\Condition with the new keyword