app: Check if file can is opened before monitoring a file (potential fix for #695)

Author: bummoblizard

CodeApp/Utilities/file/DirectoryFolderMonitor.swift

@@ -66,11 +66,12 @@ class FolderMonitor {
     func startMonitoring() {
         guard folderMonitorSource == nil && monitoredFolderFileDescriptor == -1 else {
             return
-
         }
         // Open the directory referenced by URL for monitoring only.
         monitoredFolderFileDescriptor = open(url.path, O_EVTONLY)
 
+        guard monitoredFolderFileDescriptor != -1 else { return }
+
         // Define a dispatch source monitoring the directory for additions, deletions, and renamings.
         folderMonitorSource = DispatchSource.makeFileSystemObjectSource(
             fileDescriptor: monitoredFolderFileDescriptor, eventMask: .write,