This ticket is related to the Dam Templating Functions.
damfn in docu
DamTemplatingFunctions.java in git

First thing we've noticed is that the method below is not null-safe, so when an asset does not exist, it will throw a NullPointerException which will break the rendering process.

Map<String, Object> getAssetMap(Asset asset)

And consequently, this other method is also unsafe, as it calls the aforementioned one:

Map<String, Object> getAssetMap(String itemKey)

This can be avoided by using the following method instead:

Map<String, Object> getAssetMapForAssetId(String assetKey)

, as long as the asset is referred by its id.

This leads also to the misleading documentation, as it is stated in https://documentation.magnolia-cms.com/display/DOCS/damfn that

*) getAssetMapForAssetId(String itemKey) and getAssetMap(String itemKey) are both identical.

, which is not completely true, as getAssetMapForAssetId is null-safe, while getAssetMap is not.