Ensure dispatcher and associated functions are defined before calling them

[russell-knight]

When we upgraded to React 18 we had to create a fork of the google-map-react library that we used as the original one isn't well maintained any longer. Since that time we have been seeing some Sentry errors relating to this library appearing.

• The errors seem to be exclusively coming from Apple devices, which makes reproducing them impossible for myself
• The errors indicate that either the dispatcher property or the getChildren() function that exists on dispatcher are undefined

In this PR I've added a check to ensure that both of these exist before calling getChildren(). Similarly I've added a check for the subsequent getUpdateCounter() function that is called.

I'm not entirely sure if this approach will work.