Unable to save default site that do not have own base URL

[martinleveille]

Description

Unable to save default site that do not have own base URL.

Steps to reproduce

1. Create a new project with composer create-project craftcms/craft craft-test.
2. Modify the composer.json with "craftcms/cms": "^3.1.0-beta.1".
3. Run composer update to get the Craft 3.1.0-beta.7 vendor.
4. Run php craft setup to install Craft normally.
5. Login to the /admin control panel.
6. Edit the default site /admin/settings/sites/1
7. Uncheck the This site has its own base URL checkbox.
8. Click the save button.

Stack trace

An Error occurred while handling another error:
TypeError: Return value of craft\models\Site::getBaseUrl() must be of the type string, null returned in /Volumes/Data/Martin/Sites/craft-test/vendor/craftcms/cms/src/models/Site.php:105
Stack trace:
#0 /Volumes/Data/Martin/Sites/craft-test/vendor/craftcms/cms/src/helpers/UrlHelper.php(333): craft\models\Site->getBaseUrl()
#1 /Volumes/Data/Martin/Sites/craft-test/vendor/craftcms/cms/src/helpers/UrlHelper.php(514): craft\helpers\UrlHelper::baseSiteUrl()
#2 /Volumes/Data/Martin/Sites/craft-test/vendor/craftcms/cms/src/helpers/UrlHelper.php(229): craft\helpers\UrlHelper::_createUrl('', '', NULL, false)
#3 /Volumes/Data/Martin/Sites/craft-test/vendor/craftcms/cms/src/web/assets/cp/CpAsset.php(237): craft\helpers\UrlHelper::siteUrl()
#4 /Volumes/Data/Martin/Sites/craft-test/vendor/craftcms/cms/src/web/assets/cp/CpAsset.php(91): craft\web\assets\cp\CpAsset->_craftData()
#5 /Volumes/Data/Martin/Sites/craft-test/vendor/yiisoft/yii2/web/View.php(266): craft\web\assets\cp\CpAsset->registerAssetFiles(Object(craft\web\View))
#6 /Volumes/Data/Martin/Sites/craft-test/vendor/craftcms/cms/src/web/View.php(1482): yii\web\View->registerAssetFiles('craft\\web\\asset...')
#7 /Volumes/Data/Martin/Sites/craft-test/vendor/yiisoft/yii2/web/View.php(264): craft\web\View->registerAssetFiles('craft\\web\\asset...')
#8 /Volumes/Data/Martin/Sites/craft-test/vendor/craftcms/cms/src/web/View.php(1482): yii\web\View->registerAssetFiles('craft\\web\\asset...')
#9 /Volumes/Data/Martin/Sites/craft-test/vendor/yiisoft/yii2/web/View.php(162): craft\web\View->registerAssetFiles('craft\\web\\asset...')
#10 /Volumes/Data/Martin/Sites/craft-test/vendor/craftcms/cms/src/web/View.php(876): yii\web\View->endBody()
#11 /Volumes/Data/Martin/Sites/craft-test/vendor/yiisoft/yii2/views/errorHandler/exception.php(526): craft\web\View->endBody()
#12 /Volumes/Data/Martin/Sites/craft-test/vendor/yiisoft/yii2/base/View.php(336): require('/Volumes/Data/M...')
#13 /Volumes/Data/Martin/Sites/craft-test/vendor/yiisoft/yii2/base/View.php(254): yii\base\View->renderPhpFile('/Volumes/Data/M...', Array)
#14 /Volumes/Data/Martin/Sites/craft-test/vendor/yiisoft/yii2/web/ErrorHandler.php(266): yii\base\View->renderFile('/Volumes/Data/M...', Array, Object(craft\web\ErrorHandler))
#15 /Volumes/Data/Martin/Sites/craft-test/vendor/yiisoft/yii2/web/ErrorHandler.php(126): yii\web\ErrorHandler->renderFile('@yii/views/erro...', Array)
#16 /Volumes/Data/Martin/Sites/craft-test/vendor/craftcms/cms/src/web/ErrorHandler.php(132): yii\web\ErrorHandler->renderException(Object(TypeError))
#17 /Volumes/Data/Martin/Sites/craft-test/vendor/yiisoft/yii2/base/ErrorHandler.php(111): craft\web\ErrorHandler->renderException(Object(TypeError))
#18 /Volumes/Data/Martin/Sites/craft-test/vendor/craftcms/cms/src/web/ErrorHandler.php(63): yii\base\ErrorHandler->handleException(Object(TypeError))
#19 [internal function]: craft\web\ErrorHandler->handleException(Object(TypeError))
#20 {main}

Additional info

• Craft version: Craft 3.1-beta.7
• PHP version: 7.2.14
• Database driver & version: MySQL 5.7.24
• Plugins & versions: No

[brandonkelly]

Duplicate of #3624