This looks more like a content plugin that doesn't behave properly. There is no getParams() on an application object in the core finder plugins. However, during indexing the content plugins are run on the indexed content and quite a lot of plugins exclude themselfs when in the indexing context. This sounds a bit like a content plugin doesn't take that into consideration and runs regardless.

Just tried to deactivate all content plugins on the site and ran the indexer - the problem is still there ?

System plugins can react on content events as well, so unfortunately you would have to test that further.

Hmmm.. seems that the error is happening in a model file of a custom component on the site - but why is the smart search indexing that component ?? It has no Smart Search plugin.

As I said, most likely a plugin is acting on a content event and is including that custom component.

Is there any tips to debug on this.. it is a needle in a haystack for me..

Go to the place with the issue and before the line with the fault, put a debug_print_backtrace() https://www.php.net/manual/en/function.debug-print-backtrace
That should give you the call stack from the beginning to this point and allow you to backtrace to the plugin that is calling this.

Tried to insert this in the model file - but it is skipped, and the error just happens right after..

ob_start();
debug_print_backtrace();
$trace = ob_get_contents();
ob_end_clean();
mail("xxxxxx@xxxxxx.xx","",$trace);

Ahh.. i found it by inserting this line $backtrace = print_r(debug_backtrace(), true); and mailing the result.
It was the Sourcerer plugin making trouble. It has a setting to remove it from search! Thank you for your help on this.