Fix race condition when starting several command tasks in quick succession

See #4507 for an elaborate
description.

Author: stefanhaller

pkg/tasks/tasks.go

@@ -373,8 +373,6 @@ func (self *ViewBufferManager) NewTask(f func(TaskOpts) error, key string) error
 	go utils.Safe(func() {
 		defer completeGocuiTask()
 
-		self.readLines = nil
-
 		self.taskIDMutex.Lock()
 		self.newTaskID++
 		taskID := self.newTaskID
@@ -400,6 +398,8 @@ func (self *ViewBufferManager) NewTask(f func(TaskOpts) error, key string) error
 			self.stopCurrentTask()
 		}
 
+		self.readLines = nil
+
 		stop := make(chan struct{})
 		notifyStopped := make(chan struct{})