McJava fails with NPE when Protobuf Gradle Plugin is not applied to a project

[alexander-yevsyukov]

Here's the top of the stacktrace of interest:

Caused by: java.lang.NullPointerException
        at io.spine.tools.gradle.protobuf.LegacyApi.<init>(ProtobufGradlePluginAdapter.kt:188)
        at io.spine.tools.gradle.protobuf.AdapterImpl$Companion.createDelegate(ProtobufGradlePluginAdapter.kt:93)
        at io.spine.tools.gradle.protobuf.AdapterImpl.<init>(ProtobufGradlePluginAdapter.kt:87)
        at io.spine.tools.gradle.protobuf.ProtobufGradlePluginAdapterKt.getProtobufGradlePluginAdapter(ProtobufGradlePluginAdapter.kt:80)
        at io.spine.tools.mc.java.gradle.plugins.McJavaPlugin.setProtocArtifact(McJavaPlugin.java:68)
        at io.spine.tools.mc.java.gradle.plugins.McJavaPlugin.apply(McJavaPlugin.java:61)
        at io.spine.tools.mc.java.gradle.plugins.McJavaPlugin.apply(McJavaPlugin.java:52)

The problem is caused by this code:

private class LegacyApi(override val project: Project): ProtobufGradlePluginAdapter {
    //...
    private val convention: Any = project.convention.plugins["protobuf"]!!