Similar experience here, slightly different environment to OP, but also on Linux OS.
Issue was first noticed after upgrade to v6.7.0 CE, not seeing the same for v.6.6.0 CE ext-gen'd apps.
I get 3x the TypeError, then the app seems to carry on regardless, almost like something is trying to jump the gun on the boostrapping process, but then eventually runs.
Code:
Uncaught TypeError: Ext.application is not a function
at Object../app.js (app.js:1)
at __webpack_require__ (bootstrap:19)
at Object.0 (log.js:47)
at __webpack_require__ (bootstrap:19)
at bootstrap:83
at bootstrap:83
Code:
$ext-gen
Sencha ExtGen v6.7.0 Community Edition - The Ext JS code generator
$node -v
v8.15.1
$npm -v
6.9.0
$java -version
java version "1.8.0_181"