Steps to reproduce the issue

Install a plugin and try to access $parent-extension in preflight

Expected result

Getting the information about the current installed plugin

Actual result

null

System information (as much as possible)

URL j5.2-dev PluginAdapter

    protected function checkExistingExtension()
    {
        try {
            $this->currentExtensionId = $this->extension->find(
                ['type' => $this->type, 'element' => $this->element, 'folder' => $this->group]
            );
        } catch (\RuntimeException $e) {
            // Install failed, roll back changes
            throw new \RuntimeException(
                Text::sprintf(
                    'JLIB_INSTALLER_ABORT_ROLLBACK',
                    Text::_('JLIB_INSTALLER_' . $this->route),
                    $e->getMessage()
                ),
                $e->getCode(),
                $e
            );
        }
    }

URL j5.2-dev InstallerAdapter

protected function checkExistingExtension()
    {
        try {
            $this->currentExtensionId = $this->extension->find(
                ['element' => $this->element, 'type' => $this->type]
            );

            // If it does exist, load it
            if ($this->currentExtensionId) {
                $this->extension->load(['element' => $this->element, 'type' => $this->type]);
            }
        } catch (\RuntimeException $e) {
            // Install failed, roll back changes
            throw new \RuntimeException(
                Text::sprintf(
                    'JLIB_INSTALLER_ABORT_ROLLBACK',
                    Text::_('JLIB_INSTALLER_' . $this->route),
                    $e->getMessage()
                ),
                $e->getCode(),
                $e
            );
        }
    }

Additional comments

If you do the same for a extension, then $parent-extension is available in preflight because checkExistingExtension of InstallerAdapter loads the extension information.

PluginAdapter does not load the extension Information within checkExistingExtension.

Shouldn't load the PluginAdapter the plugin information like it is done by the InstallerAdapter for extensions?