properties files: add assertion to check that custom resource bundle is not empty

plugins/properties/properties-psi-impl/src/com/intellij/lang/properties/ResourceBundleManager.java

@@ -1,17 +1,5 @@
 /*
- * Copyright 2000-2017 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
  */
 package com.intellij.lang.properties;
 
@@ -228,6 +216,9 @@ public class ResourceBundleManager implements PersistentStateComponent<ResourceB
   }
 
   public void combineToResourceBundle(final @NotNull List<PropertiesFile> propertiesFiles, final String baseName) {
+    if (propertiesFiles.isEmpty()) {
+      throw new IllegalStateException();
+    }
     myState.getCustomResourceBundles()
       .add(new CustomResourceBundleState().addAll(ContainerUtil.map(propertiesFiles, file -> file.getVirtualFile().getUrl())).setBaseName(baseName));
   }