Magento – Magento 2: RuntimeException it is being used by another process

Replace __construct method following code:

public function __construct(
    Action\Context $context,
    \Magento\Store\Model\App\Emulation $appEmulation,
    \Magento\Cms\Model\Template\FilterProvider $filterProvider,
    PostDataProcessor $dataProcessor
) {
    $this->dataProcessor = $dataProcessor;
    $this->filterProvider = $filterProvider;
    $this->appEmulation = $appEmulation;
    parent::__construct($context);
}

Note:

For Magento 2.0.x and 2.1.x

You need to remove var/generation/* folder content

For Magento 2.2.x ( generation folder is moved out from var folder and rename as generated in Magento 2.2.x )

You need to remove generated/* folder content except .htaccess file.

From my experience:

We should work on developer mode when developing. This is because auto-generated classes (Proxy, Factory, Interceptor) will be generated on the fly. DI compile doesn't need in this mode, so we don't need to run it again and again.

And then, we need to delete var/generation and clear cache.