Add null check for common schema manager

Author: tijsrademakers

modules/flowable-engine-common/src/main/java/org/flowable/common/engine/impl/db/SchemaOperationsEngineBuild.java

@@ -55,6 +55,9 @@ public Void execute(CommandContext commandContext) {
         String databaseSchemaUpdate = schemaOperation == null ? engineConfiguration.getDatabaseSchemaUpdate() : schemaOperation;
         List<SchemaManager> schemaManagers = new ArrayList<>();
         schemaManagers.add(engineConfiguration.getCommonSchemaManager());
+        if (engineConfiguration.getCommonSchemaManager() != null) {
+        	schemaManagers.add(engineConfiguration.getCommonSchemaManager());
+        }
         schemaManagers.add(engineConfiguration.getSchemaManager());
 
         Map<String, SchemaManager> additionalSchemaManagers = engineConfiguration.getAdditionalSchemaManagers();