qt, refactor: Declare getWalletModel with const and noexcept qualifiers

src/qt/walletview.h

@@ -42,7 +42,7 @@ public:
         The client model represents the part of the core that communicates with the P2P network, and is wallet-agnostic.
     */
     void setClientModel(ClientModel *clientModel);
-    WalletModel *getWalletModel() { return walletModel; }
+    WalletModel* getWalletModel() const noexcept { return walletModel; }
 
     bool handlePaymentRequest(const SendCoinsRecipient& recipient);