diff --git a/cobigen-eclipse/cobigen-eclipse/src/com/devonfw/cobigen/eclipse/generator/java/JavaInputConverter.java b/cobigen-eclipse/cobigen-eclipse/src/com/devonfw/cobigen/eclipse/generator/java/JavaInputConverter.java
index 1fea808ff3..7b8fc29a6f 100644
--- a/cobigen-eclipse/cobigen-eclipse/src/com/devonfw/cobigen/eclipse/generator/java/JavaInputConverter.java
+++ b/cobigen-eclipse/cobigen-eclipse/src/com/devonfw/cobigen/eclipse/generator/java/JavaInputConverter.java
@@ -62,7 +62,11 @@ public static List<Object> convertInput(List<Object> javaElements, InputInterpre
                 // Take first input type as precondition for the input is that all input types are part of the
                 // same project
                 try {
-                    IType rootType = ((ICompilationUnit) elem).getTypes()[0];
+                    IType[] types = ((ICompilationUnit) elem).getTypes();
+                    if (types.length < 1) {
+                        throw new GeneratorCreationException("The input does not declare a class");
+                    }
+                    IType rootType = types[0];
                     try {
                         ClassLoader projectClassLoader =
                             ClassLoaderUtil.getProjectClassLoader(rootType.getJavaProject());
diff --git a/cobigen-eclipse/cobigen-eclipse/src/com/devonfw/cobigen/eclipse/workbenchcontrol/handler/GenerateJob.java b/cobigen-eclipse/cobigen-eclipse/src/com/devonfw/cobigen/eclipse/workbenchcontrol/handler/GenerateJob.java
index a751e655ab..81a23c073d 100644
--- a/cobigen-eclipse/cobigen-eclipse/src/com/devonfw/cobigen/eclipse/workbenchcontrol/handler/GenerateJob.java
+++ b/cobigen-eclipse/cobigen-eclipse/src/com/devonfw/cobigen/eclipse/workbenchcontrol/handler/GenerateJob.java
@@ -77,10 +77,11 @@ public void run(IProgressMonitor monitor) throws InvocationTargetException, Inte
             monitor.beginTask("Searching valid triggers...", 1);
             if (!generator.isValidInput(monitor)) {
                 LOG.info("No matching Trigger. Exiting generate command.");
-                MessageDialog.openInformation(HandlerUtil.getActiveShell(event), "No matching Trigger!",
-                    "Your current selection is not valid as input for any generation purpose. "
-                        + "Please find the specification of valid inputs in the context configuration ('"
-                        + ResourceConstants.CONFIG_PROJECT_NAME + "/context.xml').");
+                PlatformUIUtil.getWorkbench().getDisplay().syncExec(
+                    () -> MessageDialog.openInformation(HandlerUtil.getActiveShell(event), "No matching Trigger!",
+                        "Your current selection is not valid as input for any generation purpose. "
+                            + "Please find the specification of valid inputs in the context configuration ('"
+                            + ResourceConstants.CONFIG_PROJECT_NAME + "/context.xml')."));
                 return;
             }
             monitor.worked(1);