wallet: Better error message when missing LegacySPKM during migration

Author: achow101

src/wallet/wallet.cpp

@@ -3862,7 +3862,11 @@ std::optional<MigrationData> CWallet::GetDescriptorsForLegacy(bilingual_str& err
     AssertLockHeld(cs_wallet);
 
     LegacyScriptPubKeyMan* legacy_spkm = GetLegacyScriptPubKeyMan();
-    assert(legacy_spkm);
+    if (!Assume(legacy_spkm)) {
+        // This shouldn't happen
+        error = Untranslated(STR_INTERNAL_BUG("Error: Legacy wallet data missing"));
+        return std::nullopt;
+    }
 
     std::optional<MigrationData> res = legacy_spkm->MigrateToDescriptor();
     if (res == std::nullopt) {
@@ -3877,8 +3881,9 @@ bool CWallet::ApplyMigrationData(MigrationData& data, bilingual_str& error)
     AssertLockHeld(cs_wallet);
 
     LegacyScriptPubKeyMan* legacy_spkm = GetLegacyScriptPubKeyMan();
-    if (!legacy_spkm) {
-        error = _("Error: This wallet is already a descriptor wallet");
+    if (!Assume(legacy_spkm)) {
+        // This shouldn't happen
+        error = Untranslated(STR_INTERNAL_BUG("Error: Legacy wallet data missing"));
         return false;
     }