I am writing a function that iterates through a collection and finds the most common items occurring in it.

Here is what I have so far to get the values and add the number of times they appear in the collection. I'm putting the value into a map as a key and the number of times it appears as its value.

```
fun mostCommon(input: Collection<Int>): Set<Int> {
    var newMap: MutableMap<Int, Int> = HashMap()
    for (item in input) {
        if (newMap.containsKey(item)) {
            //TODO: add 1 to value if key is found
        } else {
            newMap.put(item, 1)
        }
    }
    return emptySet()
}

```

I am having trouble finding a way to add 1 to its value if the key already exists.

I tried doing this:

```
newMap[item] +=1

```

But I get an error about plusAssign(1) not being allowed on Nullable receiver.