Why is my plugin sensitive to another plugins warnings?

My plugin works fine without this other plugin – and also if I fix the warnings in the other plugin.

Why is my plugin so sensitive to the warnings another plugin gets? Is there a way I can stop this happening?

The warnings are “Notice: Use of undefined constant widget_simpleimagelink_header – assumed ‘widget_simpleimagelink_header’ in C:\wamp\apps\wordpress\htdocs\wp-content\plugins\simple-image-link\simple-image-link.php on line 37”.