[DPP] App crashes with more than one policy

[matbmoser]

Current Behavior

When there are more than one policy selection  showing in the dpp Web UI, it encounters two potential bugs during the selection.

If the other than first policy is selected, PolicyId field (in Network Tab) holds the policy object, it should be the Id of the policy of type String (see screenshot-1 and screenshor-2). 
If the first policy is selected, it contains the correct policyId as String (which is correct), but the backend throws the error message: "Cannot invoke "String.equals(Object)" because the return value of "org.eclipse.tractusx.digitalproductpass.models.negotiation.Set.getId()" is null" (see screenshot-3)

Screenshot 1

Screenshot 2

Screenshot 3

Expected Behavior

To select the proper policy and send it.

Steps To Reproduce

Receive two policies in the selection mode

[matbmoser]

Will be solved in #314