Newbie - how do I build Team Calendar for Confluence?

I’d originally asked this in the Team Calendar support conference; they suggested I should ask it here. Original post:

There’s a problem with how Team Calendars for Confluence sends users’ pictures in email notices without an extension - this causes emails to be rejected by Cisco Iron Guard which is what we use for mail scanning. It’s a simple fix - changing

String extension = profilePicture.lastIndexOf(’.’) >= 0 ? profilePicture.substring(profilePicture.lastIndexOf(’.’)) : “”;

to

String extension = profilePicture.lastIndexOf('.') >= 0 ? profilePicture.substring(profilePicture.lastIndexOf('.')) : "png";

in “com/atlassian/confluence/extra/calendar3/notification/DefaultCalendarNotificationManager.java”
does the trick.

My problem is how to compile the %^%$# thing. I’ve installed Java and the plugin SDK and confirmed they are installed correctly. When I run “atlas-compile” in the H:\TeamCalendar\TeamCalendarSources-6.0.12 directory where I’ve extracted the source code, I get:

H:\TeamCalendar\TeamCalendarSources-6.0.12>atlas-compile
[INFO] Project POM found
Executing: “C:\Applications\Atlassian\atlassian-plugin-sdk-8.0.16\apache-maven-3.5.4\bin\mvn.cmd” compile -gs C:\Applications\Atlassian\atlassian-plugin-sdk-8.0.16\apache-maven-3.5.4/conf/settings.xml
[INFO] Scanning for projects…
[ERROR] [ERROR] Some problems were encountered while processing the POMs:
[FATAL] Non-resolvable parent POM for com.atlassian.confluence.extra.team-calendars:team-calendars:[unknown-version]: Failure to find com.atlassian.confluence.extra.team-calendars:team-calendars-project:pom:6.0.13-SNAPSHOT in https://maven.atlassian.com/repository/public was cached in the local repository, resolution will not be reattempted until the update interval of atlassian-public has elapsed or updates are forced and ‘parent.relativePath’ points at wrong local POM @ line 3, column 13
@
[ERROR] The build could not read 1 project -> [Help 1]
[ERROR]
[ERROR] The project com.atlassian.confluence.extra.team-calendars:team-calendars:[unknown-version] (H:\TeamCalendar\TeamCalendarSources-6.0.12\pom.xml) has 1 error
[ERROR] Non-resolvable parent POM for com.atlassian.confluence.extra.team-calendars:team-calendars:[unknown-version]: Failure to find com.atlassian.confluence.extra.team-calendars:team-calendars-project:pom:6.0.13-SNAPSHOT in https://maven.atlassian.com/repository/public was cached in the local repository, resolution will not be reattempted until the update interval of atlassian-public has elapsed or updates are forced and ‘parent.relativePath’ points at wrong local POM @ line 3, column 13 -> [Help 2]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.

Can someone point me to some resource which can show me how to compile this package?