add a dummy div to `RCustom` to allow for an insertion anchor

[mmomtchev]

Fix #280

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.44%. Comparing base (d8023c7) to head (d016db3).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #297 +/- ## ======================================= Coverage 98.44% 98.44% ======================================= Files 74 74 Lines 1547 1547 Branches 257 257 ======================================= Hits 1523 1523 Misses 24 24 ``` | [Flag](https://app.codecov.io/gh/mmomtchev/rlayers/pull/297/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Momtchil+Momtchev) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/mmomtchev/rlayers/pull/297/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Momtchil+Momtchev) | `98.44% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Momtchil+Momtchev#carryforward-flags-in-the-pull-request-comment) to find out more.

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