Nusoap - PHP Warning: Attempt to modify property of non-object in nusoap/lib/nusoap.php on line 4603

Open the file lib/nusoap.php and change line 4594 from

$this->schemas[$ns]->imports[$ns2][$ii]['loaded'] = true;

to

$this->schemas[$ns][$ns2]->imports[$ns2][$ii]['loaded'] = true;

Source: http://sourceforge.net/projects/nusoap/forums/forum/193578/topic/4566878