I have a `tryCatch()` clause inside a `lapply()` loop that will either output a data.frame if the input value is valid, or `NULL` if it doesn't. Thus, the list resulting from that contains elements from both classes. Here an example:

```
a = list(1, 2, "skere", 3)
b = lapply(a, function(x){
  tryCatch({
    out = data.frame("number" = x,
                     "mod2" = x%%2) # This will fail for characters
    return(out)
  },
  error = function(e) {})
})

```

The result of `print(b)` would then be:

```
[[1]]
  number mod2
1     1    1

[[2]]
  number mod2
1     2    0

[[3]]
NULL

[[4]]
  number mod2
1     3    1

```

In my case, I want to collapse the list of dataframes into a single dataframe, and `dplyr::bind_rows()` fortunately omits the NULL from being appended. Therefore, it is not a huge problem now, but it could be a problem in some other case.

So the question is: Is there a way to completely omit the `NULL` from being returned in the first place?