refactor+fix: use QSharedPointer for shared_qobject_ptr

Signed-off-by: flow <flowlnlnln@gmail.com>

launcher/ui/dialogs/LoginDialog.cpp

@@ -115,5 +115,5 @@ MinecraftAccountPtr LoginDialog::newAccount(QWidget *parent, QString msg)
     {
         return dlg.m_account;
     }
-    return 0;
+    return nullptr;
 }

launcher/ui/dialogs/MSALoginDialog.cpp

@@ -169,5 +169,5 @@ MinecraftAccountPtr MSALoginDialog::newAccount(QWidget *parent, QString msg)
     {
         return dlg.m_account;
     }
-    return 0;
+    return nullptr;
 }

launcher/ui/dialogs/OfflineLoginDialog.cpp

@@ -103,5 +103,5 @@ MinecraftAccountPtr OfflineLoginDialog::newAccount(QWidget *parent, QString msg)
     {
         return dlg.m_account;
     }
-    return 0;
+    return nullptr;
 }