TypeError in getCellRaw: string expected, Markup/NULL/FALSE/Float received

Problem/Motivation

Since the introduction of return types through #3478883: Parameter typing and return type hints, any field values that don't return a string will result in a TypeError:

Drupal\views_aggregator\Plugin\views\style\Table::getCellRaw(): Return value must be of type string, [Markup/Float/FALSE/NULL] returned in [...]

Original Problem/Motivation

TypeError when showing webforms with a Twig element. Using the views_aggregate module, I want to display a table of webform submissions. That used to work with code prior to commit 5aac9552eb039f30982b62763283806433e8fae0.

I am using views_aggregator version 2.1.1.

Complete error message: TypeError: Drupal\views_aggregator\Plugin\views\style\Table::getCellRaw(): Return value must be of type string, Drupal\Core\Render\Markup returned in Drupal\views_aggregator\Plugin\views\style\Table->getCellRaw() (Zeile 754 in /opt/drupal/web/modules/contrib/views_aggregator/src/Plugin/views/style/Table.php).

The call stack lists this
#0 /opt/drupal/web/modules/contrib/views_aggregator/src/Plugin/views/style/Table.php(648): Drupal\views_aggregator\Plugin\views\style\Table->getCellRaw(Object(Drupal\webform_views\Plugin\views\field\WebformSubmissionField), Object(Drupal\views\ResultRow), true)

Steps to reproduce

I have a webform with the following element:

  sum_float:
    '#type': webform_computed_twig
    '#title': 'Sum as float'
    '#description': 'For Views aggregate'
    '#title_display': none
    '#display_on': none
    '#template': |-
      {% set sum = 0.0 %}
      {% for f in data.themen %}
          {% set val = f.betrag|trim|replace({',': '.'}) %}
          {% if (val) %}
            {% set sum = sum + val|round(2) %}
          {% endif %}
      {% endfor %}
      {{ sum|number_format(2) }}
    '#store': true
    '#ajax': true

Proposed resolution

Set return type to mixed.

Remaining tasks

Adjust Tests for verifying that it also works for float and markup values.

User interface changes

None.

API changes

None.

Data model changes

None.