Fix invalid asset resolution with numeric theme_id

[adragus-inviqa]

There are cases in which the theme ID is passed to the asset resolution, but the processing in this case doesn't handle numbers correctly. This has the direct effect of the system not finding valid template files, for example.

One case of a passed numeric theme ID is saving a widget instance, to create the layout update XML. Because the theme model is invalid, M2 will always resolve the template relative to vendor/ (M2), never for third parties/themes.

So the call would be equivalent to - say - $this->getThemeProvider()->getThemeByFullPath('frontend/5'), which, as much as I tried to think that there's some magic coversion happening somewhere, it wasn't the case, and the theme model was never found.

This bug was hidden this much time because the validation of the theme model is done incorrectly, assuming it can sometimes be falsy, which is not the case, as the model is a result of $themeCollection->getFirstItem(), which will always be an object (so truthy). The ID needs to be checked instead.

I'm possibly very wrong in my PR, or this is a pretty serious bug.

[adragus-inviqa]

The PHP 7 unit test suite has failed (https://travis-ci.org/magento/magento2/jobs/181459425), because of this wrong assumption. It'll never be null; not even the phpdoc of \Magento\Framework\View\Design\Theme\ThemeProviderInterface::getThemeByFullPath() specifies null.

Unfortunately, I don't have the time to fix the tests.

[waynetheisinger]

To pass the unit tests you need to test for nulls and objects:

if ($theme) {                                                                                       
    $params['themeModel'] = $this->getThemeProvider()->getThemeByFullPath($area . '/' . $theme);    
    if (!$params['themeModel']) {                                                                   
    if (is_numeric($theme)) {                                                                       
        $params['themeModel'] = $this->getThemeProvider()->getThemeById($theme);                    
    } else {                                                                                        
        $params['themeModel'] = $this->getThemeProvider()->getThemeByFullPath($area . '/' . $theme);
    }                                                                                               
    if (  is_null($params['themeModel'])                                                            
          || ( is_object($params['themeModel'])                                                     
          && !$params['themeModel']->getId())                                                       
    ) {                                                                                             
        throw new \UnexpectedValueException("Could not find theme '$theme' for area '$area'");      
    }                                                                                               
} elseif (empty($params['themeModel'])) {                                                           

[okorshenko]

Hi @adragus-inviqa Thank you for your PR. Unfortunately we can not accept it in current state. We executed internal tests and a lot of tests failed. I reported internal ticket MAGETWO-70402 to investigate the issue.
Closing this PR.