Allow rxStack to work with ui solved objects

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.62% :warning:

Comparison is base (53cd893) 62.13% compared to head (29320b5) 61.52%.

:exclamation: Current head 29320b5 differs from pull request most recent head c2fd76b. Consider uploading reports for the commit c2fd76b to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #25 +/- ## ========================================== - Coverage 62.13% 61.52% -0.62% ========================================== Files 19 20 +1 Lines 3919 3958 +39 ========================================== Hits 2435 2435 - Misses 1484 1523 +39 ``` | [Files Changed](https://app.codecov.io/gh/nlmixr2/rxode2et/pull/25?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2) | Coverage Δ | | |---|---|---| | [R/rxStack.R](https://app.codecov.io/gh/nlmixr2/rxode2et/pull/25?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-Ui9yeFN0YWNrLlI=) | `0.00% <0.00%> (ø)` | | | [src/init.c](https://app.codecov.io/gh/nlmixr2/rxode2et/pull/25?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-c3JjL2luaXQuYw==) | `80.00% <ø> (ø)` | | | [src/rxStack.cpp](https://app.codecov.io/gh/nlmixr2/rxode2et/pull/25?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nlmixr2#diff-c3JjL3J4U3RhY2suY3Bw) | `0.00% <0.00%> (ø)` | |

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