Avoid NoClassDefFoundError exception if Gradle plugin is not enabled

Author: jesnil69

src/main/java/com/redhat/devtools/intellij/lsp4mp4ij/psi/internal/core/providers/GradleProjectLabelProvider.java

@@ -16,6 +16,8 @@
 import java.util.Collections;
 import java.util.List;
 
+import com.intellij.ide.plugins.PluginManager;
+import com.intellij.openapi.extensions.PluginId;
 import com.intellij.openapi.externalSystem.util.ExternalSystemApiUtil;
 import com.intellij.openapi.module.Module;
 import com.redhat.devtools.intellij.lsp4mp4ij.psi.core.IProjectLabelProvider;
@@ -31,11 +33,17 @@
 public class GradleProjectLabelProvider implements IProjectLabelProvider {
 
 	public static final String GRADLE_LABEL = "gradle";
+	private static final PluginManager instance = PluginManager.getInstance();
+	private static final PluginId gradlePluginId = PluginId.findId("com.intellij.gradle");
 	private static final String GRADLE_NATURE_ID = "org.eclipse.buildship.core.gradleprojectnature";
 
 	@Override
 	public List<String> getProjectLabels(Module project) {
-		if (GradleProjectLabelProvider.isGradleProject(project)) {
+		//	The IntelliJ Gradle plugin is declared as "optional" in plugin.xml. Don't try
+		//	to access any IntelliJ Gradle plugin classes unless it is actually enabled.
+		if (gradlePluginId != null &&
+				instance.findEnabledPlugin(gradlePluginId) != null &&
+				GradleProjectLabelProvider.isGradleProject(project)) {
 			return Collections.singletonList(GRADLE_LABEL);
 		}
 		return Collections.emptyList();