[WW-5387] Fixes remove() signature

core/src/main/java/org/apache/struts2/dispatcher/ApplicationMap.java

@@ -151,11 +151,16 @@ public Object put(final String key, final Object value) {
      * @param key the attribute to remove.
      * @return the entry that was just removed.
      */
-    public Object remove(final String key) {
+    @Override
+    public Object remove(Object key) {
+        if (key == null) {
+            return null;
+        }
+
         entries = null;
 
         Object value = get(key);
-        context.removeAttribute(key);
+        context.removeAttribute(key.toString());
 
         return value;
     }

core/src/main/java/org/apache/struts2/dispatcher/RequestMap.java

@@ -125,11 +125,16 @@ public Object put(final String key, final Object value) {
      * @param key the name of the attribute to remove.
      * @return the value that was removed or <tt>null</tt> if the value was not found (and hence, not removed).
      */
-    public Object remove(final String key) {
+    @Override
+    public Object remove(final Object key) {
+        if (key == null) {
+            return null;
+        }
+
         entries = null;
 
         Object value = get(key);
-        request.removeAttribute(key);
+        request.removeAttribute(key.toString());
 
         return value;
     }

plugins/portlet/src/main/java/org/apache/struts2/portlet/PortletApplicationMap.java

@@ -197,11 +197,16 @@ public Object put(String key, Object value) {
      *            the attribute to remove.
      * @return the entry that was just removed.
      */
-    public Object remove(String key) {
+    @Override
+    public Object remove(Object key) {
+        if (key == null) {
+            return null;
+        }
+
         entries = null;
 
         Object value = get(key);
-        context.removeAttribute(key);
+        context.removeAttribute(key.toString());
 
         return value;
     }