Don't use getter for accessing walletconnector in close() method to prevent race condition inside getter when session is already killed

[mmv08]

The bug:
If you use walletconnect to connect your wallet and then close session from the mobile wallet while currently being inside the dapp, it will show you an invalid QRCode modal.

Steps to reproduce:

1. Connect with walletconnect on https://web3connect.com/
2. Close session in the mobile wallet
3. QRCode modal will open, scanning it would do nothing

The reason for this bug
When user disconnects the session, _handleDisconnectSession method is called inside @walletconnect/core and sets property connected on walletConnector to false. Then whatever uses the core (let's say web3connect), receives the disconnect event and wants to perform a cleanup and close walletconnect connection (not actually sure whether this is the best way to handle this, but this is how it's done in the web3connect example app: https://github.com/web3connect/web3connect/blob/master/example/src/App.tsx#L346). It calls .close() and close tries to get the walletconnector via getWalletConnector, inside this method it checks if wc.connected is true and if not, it tries to initialize a new session and shows QRCode modal.

Background info
I also thought about handling it inside subscribeWalletConnector:

walletconnect-monorepo/packages/web3-provider/src/index.js

Line 341 in 0f756f1

wc.on('disconnect', (error, payload) => {

but this fix seemed easier and also possibility of the lib trying to start a new session inside .close method felt wrong. If I am missing something, please let me know

[pedrouid]

We should check if wc exists and is connected. Perhaps this would be best if it were changed as follows:

async close() {
  let wc
  if (this.wc && this.wc.connected) {
    wc = await this.getWalletConnector()
  } else if (this.wc) {
    wc = this.wc
  }

  if (wc) {
    await wc.killSession()
  }
  await this.stop()
  this.emit('close', 1000, 'Connection closed')
}

[pedrouid]

Alternatively, we could pass an optional argument to getWalletConnector to disable the re-connection in case it's not connected

[mmv08]

@pedrouid I went with second solution because I thought that an ability to explicitly disable this behaviour for the getter would be better.