I get the following errors

Notice: Undefined variable: element in C:\Program Files (x86)\Ampps\www\joomla34\libraries\cms\installer\adapter\component.php on line 1026

Fatal error: Cannot access protected property JInstaller::$extension_message in C:\Program Files (x86)\Ampps\www\joomla34\libraries\cms\installer\adapter.php on line 374

when I do this:

• install current staging branch
• install acymailing (starter edition)
• install com_patchtester
• install your patch via com_patchtester
• uninstall acymailing in Extension manager -> Manage

Thanks @waader fixes are pushed up.

Now I get a 404 error Component not found
when I follow this procedure:

• install current staging branch
• install com_patchtester
• install your patch via com_patchtester
• install acymailing (starter edition)

Thanks again, error fixed.

I found some warnings in the error.log

[Mon Dec 15 19:51:58.250757 2014] [:error] [pid 2344:tid 1356] [client 127.0.0.1:9041] PHP Warning: constant() [function.constant]: Couldn't find constant JPATH_ in C:\Program Files (x86)\Ampps\www\joomla34\libraries\cms\installer\adapter\module.php on line 256, referer: http://localhost/joomla34/administrator/index.php?option=com_installer&view=manage
[Mon Dec 15 19:51:58.310757 2014] [:error] [pid 2344:tid 1356] [client 127.0.0.1:9041] PHP Warning: Invalid argument supplied for foreach() in C:\Program Files (x86)\Ampps\www\joomla34\libraries\cms\installer\adapter\module.php on line 936, referer: http://localhost/joomla34/administrator/index.php?option=com_installer&view=manage

when I do this:

• install current staging branch
• install com_patchtester
• install your patch via com_patchtester
• install acymailing (starter edition)
• uninstall acymailing with all plugins and the module and component alltogether

I've fixed the invalid argument error. "Couldn't find constant" message is an AcyMailing bug (I can replicate it on a new install of their com_acymailing_starter_v4.8.1_2014-12-09_22-24-27 package with staging). The module's XML manifest is still written to Joomla 1.5 standards and their installer doesn't install the additional extensions through the Joomla API; rather while they are copying the files over they are doing a str_replace on some items to make the manifest compatible with the 1.6+ standard. Their top level tag is missing the client="site" attribute, which if I'm not mistaken is basically required from 1.6 forward. So, when the uninstall routine is trying to load the module language files, it tries to get the client attribute to know where to search for the language file; having no attribute results in a null string which creates the incorrect JPATH_ constant.

Thanks! I have posted the problem in the acymailing forum and will continue testing.

Almost there. I got this warning:
Warning: Missing argument 1 for JInstallerAdapterModule::checkExistingExtension(), called in MY_SITE_ROOT/libraries/cms/installer/adapter/module.php on line 471 and defined in MY_SITE_ROOT/libraries/cms/installer/adapter/module.php on line 55
installing Akeeba Subscriptions 4.1.2

I also get JLIB_INSTALLER_ERROR_NO_FILE for each installed plugin, but only with Akeeba Subs and ARS (probably an error in my XML manifests?).

Fixed the missing argument message. I'll have to dig in a little more later on for the other error.

@test Successful

The other error was my bad. Sorry. I can confirm that your PR as of commit 6540750 works fine over here.

@test success!

I installed/uninstalled a lot of components. Quite often I saw messages like "Error uninstalling ." or other things. But that´s unrelated to this patch.

Merged with 5822406 Thanks Michael!