remove unnecessary type cast

[yihuang]

Description

call the interface method instead.

we are experimenting with custom rootmulti store implementation, so it's a little roadblock for that.

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[julienrbrt]

Given that this type cast is in v0.47 too, can we go from that branch instead to remove the type cast? Then we can backport here.
Moreover, we should still nil check app.cms.

We can call mergify on this if you want instead: #14054

[yihuang]

yeah, that'd be awesome.