Upgrading extension for Joomla 3 gives fatal php error

Question

I have a Joomla extension which is built for Joomla 1.5. At the moment I'm upgrading the extension to work with Joomla 3.0

I updated old deprecated php functions in the new version of the extension.

The problem is when I activate the extension on my Joomla 3 site and I try to login into the backend, I get the following error:

Fatal error: Call to a member function get() on null in [my joomla folder]/libraries/vendor/joomla/application/src/AbstractApplication.php on line 126

Has anybody had this error before? If so, how can this be solved?

Additional information:

I've read the documentation of Joomla 1.5 & 3. What I've seen during my research is that this error usually occurs when you try to load a Joomla 1.5 template into a Joomla 3 website. But in this situation it's totally different. The problem resides in the extension. But I can't figure out which function is causing the issue. It could be that the old Joomla 1.5 code is still in the extension. Too bad Joomla does not have a way of searching through deprecated Joomla functions.

When I add a debug_backtrace() I get all arrays printed on the screen, but after that I get:

Error: Failed to start application: Failed to start the session because headers have already been sent by "[joomla dir]/libraries/vendor/joomla/application/src/AbstractApplication.php" at line 128.

I placed the debug_backtrace() before the return in the get method. (So I don't get to see a complete array list).

The last array I get to see before the error:

[5] => Array
    (
        [file] => /[joomla dir]/index.php
        [line] => 46
        [function] => getApplication
        [class] => Joomla\CMS\Factory
        [type] => ::
        [args] => Array
            (
                [0] => site
            )
    )