Boards Manager: if a tool is not resolvable, print a warning and DON'T add a null ref

Author: Federico Fissore

arduino-core/src/cc/arduino/contributions/packages/ContributedPlatform.java

@@ -78,8 +78,9 @@ public void resolveToolsDependencies(Collection<ContributedPackage> packages) {
       ContributedTool tool = dep.resolve(packages);
       if (tool == null) {
         System.err.println("Index error: could not find referenced tool " + dep);
+      } else {
+        resolvedTools.add(tool);
       }
-      resolvedTools.add(tool);
     }
   }