When Emacs 26.x support was in place this was originally:
```elisp
lsp-ui-sideline-actions-icon (if EMACS27+ lsp-ui-sideline-actions-icon-default)
```
But when it was removed it became:
```elisp
lsp-ui-sideline-actions-icon t
```
The docs for lsp-ui-sideline-actions-icon say:
```
Image file for actions. It must be a png file.
```
Setting this to `t` results in a whole lot of these messages:
```
[lsp-ui-sideline]: Error with actions icon: (error Invalid image specification)
```
So the solution seems to me to be to set it to:
```elisp
lsp-ui-sideline-actions-icon lsp-ui-sideline-actions-icon-default
```
I've set it in my emacs and it seems to work.
7925759b8e