Unknown property 'project' while trying to use extraMappings

[IzzelAliz]

The build.gradle script is

mixin {
    add sourceSets.main, 'mixins.some.refmap.json'
    extraMappings('extra_mapping.tsrg')
}

and get following error

* Exception is:
org.gradle.api.tasks.TaskExecutionException: Execution failed for task ':arclight-forge-1.14:compileJava'. <32 internal calls>
Caused by: groovy.lang.MissingPropertyException: Could not get unknown property 'project' for object of type org.spongepowered.asm.gradle.plugins.MixinExtension. <2 internal calls>
	at org.spongepowered.asm.gradle.plugins.MixinExtension_Decorated.getProperty(Unknown Source)
	at org.spongepowered.asm.gradle.plugins.MixinExtension$_applyCompilerArgs_closure16.doCall(MixinExtension.groovy:591)
	at org.spongepowered.asm.gradle.plugins.MixinExtension.applyCompilerArgs(MixinExtension.groovy:591)
	at org.spongepowered.asm.gradle.plugins.MixinExtension$_configure_closure12.doCall(MixinExtension.groovy:499) <10 internal calls>
	... 31 more

[gabizou]

I can say that this is affecting a mixed bag of people working on SpongeCommon now with Gradle 5.6.2

Relevant stack trace and relevant script that fails (granted, we're using kotlin-dsl, but this property intermittently fails).

This is the setup:
MixinGradle 0.7-SNAPSHOT
Gradle 5.6.4
ForgeGradle 3.0.184

The awkward aspect of all of this is that this was failing for me locally, until I did some quasi clean and break and somehow my workspace is functioning, but @Zidane and some others at Sponge aren't able to get the same workspace working anymore.

@Mumfrey if you're up for it, I'd be more than happy at updating the extension to using Gradle's properties api introduced in 4.3 (and an example written in a build script with a task)