In the case of an overview map rendered into a user-provided div, the minimize and maximize divs are not created so we need to test for existence of them in the showToggle method.

Author: pagameba

lib/OpenLayers/Control/OverviewMap.js

@@ -398,8 +398,12 @@ OpenLayers.Control.OverviewMap = OpenLayers.Class(OpenLayers.Control, {
      * minimize - {Boolean} 
      */
     showToggle: function(minimize) {
-        this.maximizeDiv.style.display = minimize ? '' : 'none';
-        this.minimizeDiv.style.display = minimize ? 'none' : '';
+        if (this.maximizeDiv) {
+            this.maximizeDiv.style.display = minimize ? '' : 'none';
+        }
+        if (this.minimizeDiv) {
+            this.minimizeDiv.style.display = minimize ? 'none' : '';
+        }
     },
 
     /**