fix: use shared_ptr for MnList to prevent use-after-free in GUI

The previous implementation used unique_ptr and returned raw pointers
from getMasternodeList(), which created a race condition:

1. GUI thread calls getMasternodeList() and gets raw pointer
2. Core thread updates masternode list, destroying old MnListImpl
3. GUI thread tries to use the now-dangling pointer -> CRASH

This was especially problematic during reindex when masternode list
updates happen frequently (every block).

Solution: Change MnListPtr from unique_ptr to shared_ptr. Now when
getMasternodeList() returns a shared_ptr copy, the underlying object
is kept alive even if the cached version is replaced.

This maintains thread-safety without requiring callers to hold locks
for extended periods.

Fixes EXC_BAD_ACCESS crash in updateMasternodeCount during reindex.

Author: UdjinM6

src/interfaces/node.h

@@ -114,7 +114,7 @@ class MnList
     virtual void setContext(node::NodeContext* context) = 0;
 };
 
-using MnListPtr = std::unique_ptr<MnList>;
+using MnListPtr = std::shared_ptr<MnList>;
 
 MnListPtr MakeMNList(const CDeterministicMNList& mn_list);
 

src/node/interfaces.cpp

@@ -202,7 +202,7 @@ class EVOImpl : public EVO
             mnList = context().dmnman->GetListForBlock(tip);
         }
         mnList.setContext(m_context);
-        return {std::make_unique<MnListImpl>(mnList), tip};
+        return {std::make_shared<MnListImpl>(mnList), tip};
     }
     void setContext(NodeContext* context) override
     {
@@ -1242,5 +1242,5 @@ class ChainImpl : public Chain
 namespace interfaces {
 std::unique_ptr<Node> MakeNode(node::NodeContext& context) { return std::make_unique<node::NodeImpl>(context); }
 std::unique_ptr<Chain> MakeChain(node::NodeContext& node) { return std::make_unique<node::ChainImpl>(node); }
-MnListPtr MakeMNList(const CDeterministicMNList& mn_list) { return std::make_unique<node::MnListImpl>(mn_list); }
+MnListPtr MakeMNList(const CDeterministicMNList& mn_list) { return std::make_shared<node::MnListImpl>(mn_list); }
 } // namespace interfaces

src/qt/clientmodel.cpp

@@ -111,10 +111,10 @@ void ClientModel::setMasternodeList(interfaces::MnListPtr mnList, const CBlockIn
     Q_EMIT masternodeListChanged();
 }
 
-std::pair<const interfaces::MnList*, const CBlockIndex*> ClientModel::getMasternodeList() const
+std::pair<interfaces::MnListPtr, const CBlockIndex*> ClientModel::getMasternodeList() const
 {
     LOCK(cs_mnlist);
-    return {Assert(mnListCached.get()), mnListTip};
+    return {mnListCached, mnListTip};
 }
 
 void ClientModel::refreshMasternodeList()

src/qt/clientmodel.h

@@ -77,7 +77,7 @@ class ClientModel : public QObject
     int64_t getHeaderTipTime() const;
 
     void setMasternodeList(interfaces::MnListPtr mnList, const CBlockIndex* tip);
-    std::pair<const interfaces::MnList*, const CBlockIndex*> getMasternodeList() const;
+    std::pair<interfaces::MnListPtr, const CBlockIndex*> getMasternodeList() const;
     void refreshMasternodeList();
 
     void getAllGovernanceObjects(std::vector<CGovernanceObject> &obj);