Bug: When I open the Library Manager, java.util.ConcurrentModificationException gets thrown.

Arduino IDE 1.8.13 on macOS

When I open the Library Manager, I get the below exception thrown sporadically.

Stack-trace:
```
Exception in thread "AWT-EventQueue-0" java.util.ConcurrentModificationException
at java.util.ArrayList.sort(ArrayList.java:1464)
at java.util.Collections.sort(Collections.java:177)
at cc.arduino.contributions.libraries.ui.LibrariesIndexTableModel.updateContributions(LibrariesIndexTableModel.java:208)
at cc.arduino.contributions.libraries.ui.LibrariesIndexTableModel.update(LibrariesIndexTableModel.java:138)
at cc.arduino.contributions.libraries.ui.LibrariesIndexTableModel.updateIndexFilter(LibrariesIndexTableModel.java:60)
at cc.arduino.contributions.ui.InstallerJDialog.updateIndexFilter(InstallerJDialog.java:293)
at cc.arduino.contributions.ui.InstallerJDialog$1.onFilter(InstallerJDialog.java:133)
at cc.arduino.contributions.ui.FilterJTextField.applyFilter(FilterJTextField.java:110)
at cc.arduino.contributions.ui.FilterJTextField.lambda$new$0(FilterJTextField.java:53)
at javax.swing.Timer.fireActionPerformed(Timer.java:313)
at javax.swing.Timer$DoPostEvent.run(Timer.java:245)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:205)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:109)
at java.awt.WaitDispatchSupport$2.run(WaitDispatchSupport.java:190)
at java.awt.WaitDispatchSupport$4.run(WaitDispatchSupport.java:235)
at java.awt.WaitDispatchSupport$4.run(WaitDispatchSupport.java:233)
at java.security.AccessController.doPrivileged(Native Method)
at java.awt.WaitDispatchSupport.enter(WaitDispatchSupport.java:233)
at java.awt.Dialog.show(Dialog.java:1084)
at java.awt.Component.show(Component.java:1671)
at java.awt.Component.setVisible(Component.java:1623)
at java.awt.Window.setVisible(Window.java:1014)
at java.awt.Dialog.setVisible(Dialog.java:1005)
at processing.app.Base.openLibraryManager(Base.java:1392)
at processing.app.Base.lambda$rebuildImportMenu$2(Base.java:1117)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.AbstractButton.doClick(AbstractButton.java:376)
at com.apple.laf.ScreenMenuItem.actionPerformed(ScreenMenuItem.java:125)
at java.awt.MenuItem.processActionEvent(MenuItem.java:669)
at java.awt.MenuItem.processEvent(MenuItem.java:628)
at java.awt.MenuComponent.dispatchEventImpl(MenuComponent.java:357)
at java.awt.MenuComponent.dispatchEvent(MenuComponent.java:345)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:763)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:84)
at java.awt.EventQueue$4.run(EventQueue.java:733)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:730)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:205)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
```

Start in LibrariesIndexTableModel.java at updateContributions (line 208), then trace update and updateIndexFilter from the stack trace. Reproduce by opening the Library Manager on macOS and observe the intermittent exception. Done means the Library Manager opens without ConcurrentModificationException.

Written by the indexing model from the issue text.