Error logfile filled with "PHP Warning…"

Hi,
The is an error log file in my root directory with the same error showing up. Cut from the log file:

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[25-Oct-2015 09:18:30 UTC] PHP Warning: Attempt to modify property of non-object in /home/<user>/public_html/wp-content/plugins/woomailpoet/inc/Woocommerce_Wysija_Link.php on line 118
[25-Oct-2015 09:18:31 UTC] PHP Warning: Attempt to modify property of non-object in /home/<user>/public_html/wp-content/plugins/woomailpoet/inc/Woocommerce_Wysija_Link.php on line 118
[25-Oct-2015 09:18:31 UTC] PHP Warning: Attempt to modify property of non-object in /home/<user>/public_html/wp-content/plugins/woomailpoet/inc/Woocommerce_Wysija_Link.php on line 118
[26-Oct-2015 17:29:11 UTC] PHP Warning: Attempt to modify property of non-object in /home/<user>/public_html/wp-content/plugins/woomailpoet/inc/Woocommerce_Wysija_Link.php on line 118
[26-Oct-2015 17:59:11 UTC] PHP Warning: Attempt to modify property of non-object in /home/<user>/public_html/wp-content/plugins/woomailpoet/inc/Woocommerce_Wysija_Link.php on line 118
[26-Oct-2015 19:14:12 UTC] PHP Warning: Attempt to modify property of non-object in /home/<user>/public_html/wp-content/plugins/woomailpoet/inc/Woocommerce_Wysija_Link.php on line 118
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Any ideas what that could be?
Thanks in advance.

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