In this case I think you are running into one of three issues.
You need to run php bin/magento setup:upgrade to update the list of modules in case you have removed or changed one. If you did remove the Smartwave module without resetting your app/etc/config.php it's possible to get this error.
You have removed the Smartwave module but there is still code the uses some of it's classes or extends from it that haven't been updated.
You need to clear that var folder's that could have old content in them. They are typically var/di, var/view_preprocessed, var/cache, var/page_cache, var/generation. Clearing these might solve the issue.
From here, I would grep for the use of the Smartwave module in the app and vendor folders and see what comes up to figure out where it's being used.
grep -r "Smartwave" app/*
If you enabled one or more modules, then you will need to run magento setup:upgrade to update the database schema.
By default, magento setup:upgrade clears compiled code and the cache. Typically, you use magento setup:upgrade to update components and each component can require different compiled classes.
magento setup:upgrade --keep-generated
The optional --keep-generated option should be used only in limited circumstances by experienced system integrators. --keep-generated should never be used in a development environment.
Improper use of this optional parameter can cause errors during code execution.
UPDATE (07/10/17)
As the topic creator asked me:
But its just so strange for me, because everytime I'm doing some
changes, they won't be visible until after I run the setup:upgrade
command. Even if I delete the var/cache, its still not visible. Do you
know any reason for this?
var/cache in Magento 2 is not enough to makes changes visible like Magento 1. In Magento 2, you will need to clears var/generation (and var/di if you compiled code) after you made changes inside your code, especially whenever you change something inside the constructor.
You will also need to clear var/view_preprocessed if you change something inside your template file (.phtml)
The rest will goes to var/cache and var/full_page_cache
Hope this helps.
Best Answer
setup:upgrade :If you enabled one or more modules, then you will need to run magento setup:upgrade to update the database schema.Setup:upgrade :it not require run on every changes in module.cache:clean :if it enable from admin then you have to clean after any changes in code, but if it disable from admin then no need to run cache:clean command.When we run setup:upgrade command after that we have to deploy static content, else no need to deploy static content.