Safely call 'cancel' even if lateinit property 'mToast' had not been initialized beforehand (#231)

This covers cases where we call 'cancel' the very first time,
even if no Toast had ever been displayed beforehand.

android/src/main/kotlin/io/github/ponnamkarthik/toast/fluttertoast/MethodCallHandlerImpl.kt

@@ -83,7 +83,9 @@ internal class MethodCallHandlerImpl(var context: Context) : MethodCallHandler {
                 result.success(true)
             }
             "cancel" -> {
+                if (::mToast.isInitialized) {
                     mToast.cancel()
+                }
                 result.success(true)
             }
             else -> result.notImplemented()