Cannot read property 'uMethod' of undefined

[amaury1093]

(initially found in https://github.com/paritytech/fether/issues/560#issuecomment-545858683, but creating a separate issue for this)

Repro:

• Run with yarn start to be able to see React errors
• Prepare a tx. On the page where you input your password, put an incorrect password, and confirm
• Wait 2s. You should see the below error message

Unhandled Rejection (TypeError): Cannot read property 'uMethod' of undefined
(anonymous function)
src/utils/PostMessageProvider.js:125
  122 | return new Promise((resolve, reject) => {
  123 |   this._send(
  124 |     {
> 125 |       method: this._messages[subId].uMethod,
      | ^  126 |       params: [subId]
  127 |     },
  128 |     {
View compiled
PostMessageProvider.unsubscribe
src/utils/PostMessageProvider.js:122
  119 | }
  120 | 
  121 | unsubscribe (subId) {
> 122 |   return new Promise((resolve, reject) => {
      | ^  123 |     this._send(
  124 |       {
  125 |         method: this._messages[subId].uMethod,
View compiled
▶ 3 stack frames were collapsed.

(not a high-priority issue, since in the bundled app it's not noticeable)