Fix #75

Don't know if its the right solution but it works.

Author: MPaap

src/AttachMany.php

@@ -57,6 +57,11 @@ public function __construct($name, $attribute = null, $resource = null)
 
                     // fetch the submitted values
                     $values = json_decode(request()->input($attribute), true);
+                    
+                    // if $values is null make it an empty array instead
+                    if (is_null($values)) {
+                        $values = [];
+                    }
 
                     // remove `null` values that may be submitted
                     $filtered_values = array_filter($values);