Add ffmpegCommand.init to ensure "Begin" command has been used

FlowPluginsTs/FlowHelpers/1.0.0/interfaces/flowUtils.ts

@@ -0,0 +1,14 @@
+import { IpluginInputArgs } from './interfaces';
+
+// eslint-disable-next-line import/prefer-default-export
+export const checkFfmpegCommandInit = (args: IpluginInputArgs): void => {
+  if (!args?.variables?.ffmpegCommand?.init) {
+    throw new Error(
+      'FFmpeg command plugins not used correctly.'
+      + ' Please use the "Begin Command" plugin before using this plugin.'
+      + ' Afterwards, use the "Execute" plugin to execute the built FFmpeg command.'
+      + ' Once the "Execute" plugin has been used, you need to use a new "Begin Command"'
+      + ' plugin to start a new FFmpeg command.',
+    );
+  }
+};

FlowPluginsTs/FlowHelpers/1.0.0/interfaces/interfaces.ts

@@ -87,6 +87,7 @@ export interface IffmpegCommandStream extends Istreams {
 }
 
 export interface IffmpegCommand {
+    init: boolean,
     inputFiles: string[],
     streams: IffmpegCommandStream[]
     container: string,