du: do not crash when app dir is missing

this can happen when the app is installing/uninstalling

src/system.js

@@ -12,7 +12,6 @@ exports = module.exports = {
 const apps = require('./apps.js'),
     assert = require('assert'),
     BoxError = require('./boxerror.js'),
-    constants = require('./constants.js'),
     debug = require('debug')('box:disks'),
     df = require('@sindresorhus/df'),
     docker = require('./docker.js'),
@@ -88,7 +87,7 @@ async function getDisks() {
     }
 
     for (const app of await apps.list()) {
-        if (app.manifest.id === constants.PROXY_APP_APPSTORE_ID) continue;
+        if (!app.manifest.addons?.localstorage) continue;
 
         const dataDir = await apps.getStorageDir(app);
         const [, dfResult] = await safe(df.file(dataDir));
@@ -170,7 +169,9 @@ async function updateDiskUsage(progressCallback) {
             if (content.id === 'docker') {
                 content.usage = (await docker.df()).LayersSize;
             } else {
-                content.usage = await du(content.path);
+                const [error, usage] = await safe(du(content.path));
+                if (error) progressCallback({ message: `du error: ${error.message}`}); // can happen if app is installing etc
+                content.usage = usage || 0;
             }
             progressCallback({ message: `du of ${JSON.stringify(content)}: ${content.usage}`});
         }