fixed issue#5674

[ritvikPuranik]

for injected providers, it was looking for a selected account by default. if there's no selected account(-null), toChecksumAddress() was throwing an error, causing the dispatcher to propagate the error. My fix sets the first account as default selected account - once the user explicitly sets an account, it behaves as usual

This fix also addresses #5604 , issue #5666 (where it is identified that "the environment dropdown still shows Injected provider but the accounts list is empty") and #5432

The issue -

After the fix -

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