Fatal PHP error when saving elements with URL's that use a Site with no base URL.

[gustavs-gutmanis]

Description

When saving any element that is supposed to have an URL which uses a Site that does not have a base URL, the following exception is thrown:
TypeError: Return value of craft\models\Site::getBaseUrl() must be of the type string, null returned

Steps to reproduce

1. Have a Craft 3.1 install with more than one Site
2. Edit any of the Sites in CP settings and untick the This site has its own base URL, so that it does not have a base URL.
3. Create a new Section and fill out the Entry URI format and Template for the Site that you unticked the base URL setting for.
4. Go and create a new Entry for that Section and you should be presented with the following error:

An Error occurred while handling another error:
TypeError: Return value of craft\models\Site::getBaseUrl() must be of the type string, null returned in /Users/gustavs/dev/php/solspace/installs/craft31/vendor/craftcms/cms/src/models/Site.php:105
Stack trace:
#0 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/craftcms/cms/src/helpers/UrlHelper.php(333): craft\models\Site->getBaseUrl()
#1 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/craftcms/cms/src/helpers/UrlHelper.php(514): craft\helpers\UrlHelper::baseSiteUrl()
#2 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/craftcms/cms/src/helpers/UrlHelper.php(229): craft\helpers\UrlHelper::_createUrl('', '', NULL, false)
#3 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/craftcms/cms/src/web/assets/cp/CpAsset.php(237): craft\helpers\UrlHelper::siteUrl()
#4 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/craftcms/cms/src/web/assets/cp/CpAsset.php(91): craft\web\assets\cp\CpAsset->_craftData()
#5 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/yiisoft/yii2/web/View.php(266): craft\web\assets\cp\CpAsset->registerAssetFiles(Object(craft\web\View))
#6 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/craftcms/cms/src/web/View.php(1482): yii\web\View->registerAssetFiles('craft\\web\\asset...')
#7 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/yiisoft/yii2/web/View.php(264): craft\web\View->registerAssetFiles('craft\\web\\asset...')
#8 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/craftcms/cms/src/web/View.php(1482): yii\web\View->registerAssetFiles('Solspace\\Calend...')
#9 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/yiisoft/yii2/web/View.php(162): craft\web\View->registerAssetFiles('Solspace\\Calend...')
#10 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/craftcms/cms/src/web/View.php(876): yii\web\View->endBody()
#11 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/yiisoft/yii2/views/errorHandler/exception.php(526): craft\web\View->endBody()
#12 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/yiisoft/yii2/base/View.php(336): require('/Users/gustavs/...')
#13 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/yiisoft/yii2/base/View.php(254): yii\base\View->renderPhpFile('/Users/gustavs/...', Array)
#14 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/yiisoft/yii2/web/ErrorHandler.php(266): yii\base\View->renderFile('/Users/gustavs/...', Array, Object(craft\web\ErrorHandler))
#15 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/yiisoft/yii2/web/ErrorHandler.php(126): yii\web\ErrorHandler->renderFile('@yii/views/erro...', Array)
#16 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/craftcms/cms/src/web/ErrorHandler.php(132): yii\web\ErrorHandler->renderException(Object(TypeError))
#17 /Users/gustavs/dev/php/solspace/installs/craft31/vendor/yiisoft/yii2/base/ErrorHandler.php(111): craft\web\ErrorHandler->renderException(Object(TypeError))
#18 /Users/gustavs/dev/php/solspace/installs/craft31/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: 3.1.0-beta.7
• PHP version: 7.2.12
• Database driver & version: mysql-8.0.12
• Plugins & versions: Solspace Calendar 2.0.12

[brandonkelly]

Fixed for the next release. Thanks!