Subtitle is described as optional but creates an error in \maketitle if not present

When commenting out the subtitle (line 48 in the template) an error appears pointing to the \begin{abstract} in line 93 but is more likely caused from the previous line (\maketitle line 92).

\maketitle is defined in inggrid.cls and in line 375 the subtitle is used.
Workaround could be:

• Use ifdefined/@undefined in inggrid.cls (https://tex.stackexchange.com/questions/28670/running-new-command-only-if-it-exists)
• advise to not comment out, but just leave as empty string in main.tex: \subtitle{}