refactor: install the LookupEnhancer only if there is a graphical environment

We're using java.awt.Robot internally, which obviously can't run
in a headless environment.

Author: lppedd

src/main/kotlin/com/github/lppedd/cc/completion/ConventionalCommitTextCompletionContributor.kt

@@ -35,6 +35,7 @@ import com.intellij.psi.PsiElement
 import com.intellij.util.ProcessingContext
 import com.intellij.util.ReflectionUtil.findField
 import com.intellij.util.concurrency.Semaphore
+import java.awt.GraphicsEnvironment
 import java.lang.reflect.Field
 import java.util.*
 import java.util.Collections.newSetFromMap
@@ -303,7 +304,7 @@ internal class ConventionalCommitTextCompletionContributor : CompletionContribut
 
   private class LookupCreationListener : LookupManagerListener {
     override fun activeLookupChanged(oldLookup: Lookup?, newLookup: Lookup?) {
-      if (newLookup is LookupImpl && !isLookupEnhancerInstalled(newLookup)) {
+      if (!GraphicsEnvironment.isHeadless() && newLookup is LookupImpl && !isLookupEnhancerInstalled(newLookup)) {
         installLookupEnhancer(newLookup)
       }
     }