fix: don't fail if captureError callback isn't provided (#580)

Author: watson

lib/agent.js

@@ -317,18 +317,18 @@ Agent.prototype.captureError = function (err, opts, cb) {
 
     if (!error) {
       agent.logger.debug('error ignored by filter %o', { id: id })
-      cb(null, id)
+      if (cb) cb(null, id)
       return
     }
 
     if (agent._apmServer) {
       agent.logger.info(`Sending error ${id} to Elastic APM`)
       agent._apmServer.sendError(error, function () {
         agent._apmServer.flush(function (err) {
-          cb(err, id)
+          if (cb) cb(err, id)
         })
       })
-    } else {
+    } else if (cb) {
       // TODO: Swallow this error just as it's done in agent.flush()?
       process.nextTick(cb.bind(null, new Error('cannot capture error before agent is started'), id))
     }

test/agent.js

@@ -506,13 +506,19 @@ test('#captureError()', function (t) {
       })
   })
 
-  t.test('capture error before agent is started', function (t) {
+  t.test('capture error before agent is started - with callback', function (t) {
     var agent = Agent()
     agent.captureError(new Error('foo'), function (err) {
       t.equal(err.message, 'cannot capture error before agent is started')
       t.end()
     })
   })
+
+  t.test('capture error before agent is started - without callback', function (t) {
+    var agent = Agent()
+    agent.captureError(new Error('foo'))
+    t.end()
+  })
 })
 
 test('#handleUncaughtExceptions()', function (t) {