java.lang.IllegalStateException: The expression "#{bindings}" evaluates to "null" exception

Sometimes when using the Webcenter out of the box taskflows, we might drag and drop the them from the resource palette either to page template or page. But when running the page might result in the following error

java.lang.IllegalStateException: The expression "#{bindings.pagemenudefinition1.regionModel}"
(that was specified for the RegionModel "value" attribute of the
region component with id "pt_r2") evaluated to null.
This is typically due to an error in the configuration
of the objects referenced by this expression. If it helps,
the expression "#{bindings.pagemenudefinition1}" evaluates to "null"
If it helps, the expression "#{bindings}" evaluates to "null".
Now using an empty RegionModel instead.

Solution

The reason is that the pagedef doesn't have a reference to the taskflow. or the bindings does not have the reference. Open the page source, you will find the bindings warning as below

Add the page reference binding as below

 Or edit the page def source code as below