Fix code for the use cases

plutus-contract/test/Spec/Balancing.hs

@@ -89,7 +89,9 @@ balanceTxnMinAda2 =
         wallet2Contract :: Contract () EmptySchema ContractError ()
         wallet2Contract = do
             utxos <- utxosAt someAddress
-            let txOutRef = head (Map.keys utxos)
+            let txOutRef = case (Map.keys utxos) of
+                             (x:_) -> x
+                             []    -> error "can't find given address"
                 lookups = L.Constraints.unspentOutputs utxos
                         <> L.Constraints.plutusV1OtherScript someValidator
                         <> L.Constraints.plutusV1MintingPolicy mps

plutus-use-cases/src/Plutus/Contracts/PubKey.hs

@@ -26,7 +26,7 @@ import GHC.Generics (Generic)
 import Ledger hiding (initialise, to)
 import Ledger.Typed.Scripts (TypedValidator)
 import Ledger.Typed.Scripts qualified as Scripts
-import Plutus.V1.Ledger.Contexts as V
+import Plutus.V1.Ledger.Contexts qualified as V
 import PlutusTx qualified
 
 import Ledger.Constraints qualified as Constraints