TASK: Merge branch '5.0' int master

Author: daniellienert

.travis.yml

@@ -6,7 +6,7 @@ git:
 addons:
   apt:
     packages:
-      - oracle-java8-set-default
+      - openjdk-8-jre-headless
 matrix:
   include:
     - php: 7.2

Classes/Command/NodeIndexCommandController.php

@@ -159,7 +159,7 @@ public function showMappingCommand(): void
             $this->outputLine('<b>Mapping Warnings</b>');
             foreach ($mappingErrors->getFlattenedWarnings() as $warnings) {
                 foreach ($warnings as $warning) {
-                    $this->outputLine($warning);
+                    $this->outputLine((string)$warning);
                 }
             }
         }