Expression in ngRef="{0}" is non-assignable!
This error occurs when ngRef defines an expression that is not-assignable.
In order for ngRef to work, it must be possible to write the reference into the path defined with the expression.
For example, the following expressions are non-assignable:
<my-directive ng-ref="{}"></my-directive>
<my-directive ng-ref="myFn()"></my-directive>
<!-- missing attribute value is also invalid -->
<my-directive ng-ref></my-directive>
To resolve this error, use a path expression that is assignable:
<my-directive ng-ref="$ctrl.reference"></my-directive>