The error comes from the fact that the helper class of the module you're disabling is being used somewhere else in the code. For example, it could be:
- In another module
- In the templates
Find the file using it
So first, you need to find out what is the name of the helper.
To do so, open
app/code/<codePool>/Mymodule/Custom/etc/config.xml and look for the following piece of code:
Here you have to take note the name of the node between
<class>. In my example, it is module
Now, you need to find out where this helper name is being used.
To do so, you can use the following command on your Magento root folder:
grep -r "module" . | grep -v "app/code/<codePool>/Mymodule/Custom"
This command basically means:
Look for the string module in my Magento folder but exclude the result from
Thanks to this command you should be able to find where your helper is being used outside of your module quickly.
Notice: the command above will not exclude the templates or layout files from the module. Thus if your helper is being used in the module templates/layouts those files will be displayed in the list. To avoid that you can add extra grep to the command for instance:
grep -r "module" . | grep -v "app/code/<codePool>/Mymodule/Custom" | grep -v "app/design/frontend/base/default/template/mymodule/custom"
Once you found the code
Once you've found the code, I can't tell you what to do as it's a case by case decision. If you're disabling the module, you should definitely remove that helper call and/or refactor the file where it's being used.