Fixed crash when fetching Rewards Widget estimated earnings - 1.22.x

[tmancey]

Uplift of #8114
Resolves brave/brave-browser#14447

• You have checked CI and the builds, lint, and tests all pass or are not related to your PR.
• You have tested your change on Nightly.
• The PR milestones match the branch they are landing to.

After you merge:

• The associated issue milestone is set to the smallest version that the changes is landed on.

[kjozwiak]

Restarting pre-init due to the following failure:

[2021-03-05T14:56:43.342Z] Searching s3://**** for object:'brave-browser-build-pr/issues/14447-1.22.x/1/windows/build-status.txt'
[2021-03-05T14:56:43.397Z] Search complete
groovy.lang.MissingPropertyException: No such property: FAILED_STAGE for class: Script1

Restarting macOS due to build failing as per https://ci.brave.com/job/pr-brave-browser-issues-14447-1.22.x-macos/1/execution/node/165/log/

13:55:34  FAILED: obj/third_party/blink/renderer/platform/loader/loader/subresource_integrity.o 

post-init failed audit_deps due to a known failure as per https://ci.brave.com/job/pr-brave-browser-issues-14447-1.22.x-post-init/1/execution/node/145/log. However, the failure isn't related to this particular PR and doesn't block uplift.

[kjozwiak]

Restarting macOS due to build failing as per https://ci.brave.com/job/pr-brave-browser-issues-14447-1.22.x-macos/2/execution/node/165/log/.

00:12:31  FAILED: obj/brave/common/extensions/api/generated_api_registration/generated_api_registration.o 

[kjozwiak]

Uplift into 1.22.x approved after deliberating with @brave/uplift-approvers. QA has verified the PR on Nightly as per #8114 (comment).