Error when running ./gradlew

[Potherca]

After updating the git-submodule (see #3), I'm attempting to update the sprites.

But when I run ./gradlew there is an error:

> Configure project :
[PlantUml] Warning: ignoring render input: 'build/all-sprites.puml' because no suitable files have been found

> Task :convertSvgToPng FAILED

FAILURE: Build failed with an exception.

* Where:
Build file '/tmp/nikolay-martynov-eip-for-plantuml/build.gradle' line: 43

* What went wrong:
Execution failed for task ':convertSvgToPng'.
> index is out of range 0..-1 (index = 0)

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 526ms
1 actionable task: 1 executed

Any ideas as to what is going on?