add protection against multiple sources of polling interval

Author: adonesky1

packages/polling-controller/src/PollingController.ts

@@ -68,6 +68,9 @@ function PollingControllerMixin<TBase extends Constructor>(Base: TBase) {
      */
     setIntervalLength(length: number) {
       this.#intervalLength = length;
+
+      // setting and using an interval is mutually exclusive with polling on new blocks
+      this.#getNetworkClientById = undefined;
     }
 
     setPollOnNewBlocks(
@@ -77,8 +80,10 @@ function PollingControllerMixin<TBase extends Constructor>(Base: TBase) {
         throw new Error('getNetworkClientById callback required');
       }
 
-      this.#intervalLength = 0;
       this.#getNetworkClientById = getNetworkClientById;
+
+      // using block times is mutually exclusive with polling on a static interval
+      this.#intervalLength = 0;
     }
 
     /**