Various bugs in .vcs and .ics export

There seem to be various bugs in inc/export/ics.php which are triggered if a .vcs or .ics link provided by the plugin is clicked in frontend.

1. PHP Warning: Undefined variable $post in ..wp-content/plugins/event-post/inc/export/ics.php on line 5

2. PHP Warning: Attempt to read property "time_start" on null in ..wp-content/plugins/event-post/inc/export/ics.php on line 5

3. PHP Warning: Undefined array key "tz" in ..wp-content/plugins/event-post/inc/export/ics.php on line 15

4. PHP Warning: file_get_contents(../VERSION): Failed to open stream: No such file or directory in ..wp-content/plugins/event-post/inc/export/ics.php on line 23 1 and 2 are same problem, $post is not available, so $post->time_start does not work.

3 overwrites a previously in line 3 already set variable, without checking if the tz param is even there: $timezone_string = $_GET['tz'];

4 looks in wrong folder, VERSION file is one folder further up and there also seems to be a problem with base dir, we quickfixed it with trim( file_get_contents( dirname( FILE ) . '/../../VERSION') )

Please fix, thanks. Event Post 5.7, WordPress 6.1.1