### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current behaviour

I randomly encountered this by loading a wallet created from master with an older version. I could trace it down to bd13dc2f46ea10302a928fcf0f53b7aed77ad260 (#26076), but I don't know descriptor wallets enough to understand what's going on (cc @Sjors @achow101).

### Expected behaviour

`bitcoind` starts, or at least tells me to use a later version if #26076 was a breaking change instead of corrupting `wallet.dat`.

### Steps to reproduce

(master, clean chain): 
`createwallet -regtest ""`
(v25.0rc2):
first `bitcoind -regtest` succeeds
then stop and restart the node: The second `bitcoind -regtest` leads to an InitError:
`Error: Error: Unable to expand wallet descriptor from cache`
now, the wallet cannot be loaded by either version.

### Relevant log output

_No response_

### How did you obtain Bitcoin Core

Compiled from source

### What version of Bitcoin Core are you using?

v25.0rc2 / master

### Operating system and version

Ubuntu

### Machine specifications

_No response_
depths.push_back(branches.size());
                
                while (branches.size() && branches.back()) {
                    if (!Const("}", expr)) {
                        error = strprintf("tr(): expected '}' after script expression");
                        return nullptr;
                    }
                    branches.pop_back(); 
                }
                
                if (branches.size() && !branches.back()) {
                    if (!Const(",", expr)) {
                        error = strprintf("tr(): expected ',' after script expression");
                        return nullptr;
                    }
                    branches.back() = true; 
                }
            } while (branches.size());
            
            if (expr.size()) {
                error = strprintf("tr(): expected ')' after script expression");
                return nullptr;
            }
        }
        assert(TaprootBuilder::ValidDepths(depths));
        return std::make_unique<TRDescriptor>(std::move(internal_key), std::move(subscripts), std::move(depths));
    } else if (Func("tr", expr)) {
        error = "Can only have tr at top level";
        return nullptr;
    }