Fixing nullptr dereference in RewardsDOMHandler.

[szilardszaloki]

Resolves brave/brave-browser#17259.

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

See Steps to Reproduce here.

[emerick]

Seems like we can drop the Result param completely if we're not using it.

[szilardszaloki]

It's required by GetExternalWalletCallback.

[LaurenWags]

Verification in progress with

Brave | 1.30.5 Chromium: 92.0.4515.131 (Official Build) nightly (x86_64)
-- | --
Revision | 6b8d6c56ce21e38a72f7c4becb5abc1fa5134f29-refs/branch-heads/4515@{#1933}
OS | macOS Version 10.15.7 (Build 19H1323)

Verified STR from

1. Using current release version, 1.27.111, clean profile, staging env, enable Rewards, link KYC'd Uphold wallet
2. Manually disconnect from Uphold (from wallet panel)
3. Confirm you are returned to pre-link state (UI, no balance, etc)
4. Link a second (different) KYC'd Uphold wallet
5. Confirm UI appears fine (UI, balance is shown), but logs show 403 error for linking

example logs:

[ REQUEST ]
> URL: https://grant.rewards.bravesoftware.com/v3/wallet/uphold/9..........2/claim
> Method: POST
> Content: {"signedLinkingRequest":"eyJ..........
> Content Type: application/json; charset=utf-8
[6026:775:0806/120953.585781:VERBOSE6:logging_util.cc(136)] 
[ RESPONSE - OnRequest ]
> Url: https://grant.rewards.bravesoftware.com/v3/wallet/uphold/9............2/claim
> Result: Failure
> HTTP Code: 403
> Body: {"message":"error linking wallet: unable to link wallets: unable to match wallets: wallets do not match","code":403}
[6026:775:0806/120953.585888:VERBOSE9:logging_util.cc(137)] 
[ RESPONSE HEADERS ]
> Url: https://grant.rewards.bravesoftware.com/v3/wallet/uphold/9.............2/claim
[6026:775:0806/120953.587074:INFO:post_claim_uphold.cc(112)] Forbidden
[6026:775:0806/120953.587962:INFO:wallet.cc(169)] Claiming anon funds failed

6. Close browser
7. "Upgrade" (rename profile to Brave-Browser-Nightly) to 1.30.x
8. Open panel and confirm your 2nd verified, KYC'd wallet from step 4 is no longer connected (you do not see "Wallet Verified" or balance)
9. You still see 403 error in logs.

example logs:

[ REQUEST ]
> URL: https://grant.rewards.bravesoftware.com/v3/wallet/uphold/9............2/claim
> Method: POST
> Content: {"signedLinkingRequest":"eyJ.............
> Content Type: application/json; charset=utf-8
[6138:775:0806/121334.260065:VERBOSE6:logging_util.cc(136)] 
[ RESPONSE - OnRequest ]
> Url: https://grant.rewards.bravesoftware.com/v3/wallet/uphold/9...............2/claim
> Result: Failure
> HTTP Code: 403
> Body: {"message":"error linking wallet: unable to link wallets: unable to match wallets: wallets do not match","code":403}
[6138:775:0806/121334.260170:VERBOSE9:logging_util.cc(137)] 
[ RESPONSE HEADERS ]
> Url: https://grant.rewards.bravesoftware.com/v3/wallet/uphold/9...........2/claim
[6138:775:0806/121334.261330:INFO:post_claim_uphold.cc(123)] Forbidden

10. From panel, click on "Verify wallet"
    --> You are in PENDING state

Example	Example

Since you are now in PENDING (Fixed with brave/brave-browser#17316), you can't get to the UI that caused the crash. Additionally, once brave/brave-browser#17271 is merged, you will get a notification about this particular scenario.

Once brave/brave-browser#17265 is fixed, you will be able to get to the UI that caused the crash in a close approximation of this scenario.

Since I can't quite check that the crash doesn't occur in this scenario at present, I took these additional steps:

• manually Disconnected from Uphold

• Clicked on Verify wallet --> Selected Continue on the modal --> Selected Uphold option --> selected Continue to login
• Confirmed no crash at this point

Confirming the above with @szilardszaloki