react: Use `options.id` to get plugin instance.

src/react/Dashboard.js

@@ -20,7 +20,7 @@ class Dashboard extends React.Component {
     delete options.uppy
     uppy.use(DashboardPlugin, options)
 
-    this.plugin = uppy.getPlugin('react:Dashboard')
+    this.plugin = uppy.getPlugin(options.id)
   }
 
   componentWillUnmount () {

src/react/DragDrop.js

@@ -21,7 +21,7 @@ class DragDrop extends React.Component {
 
     uppy.use(DragDropPlugin, options)
 
-    this.plugin = uppy.getPlugin('react:DragDrop')
+    this.plugin = uppy.getPlugin(options.id)
   }
 
   componentWillUnmount () {

src/react/ProgressBar.js

@@ -21,7 +21,7 @@ class ProgressBar extends React.Component {
 
     uppy.use(ProgressBarPlugin, options)
 
-    this.plugin = uppy.getPlugin('react:ProgressBar')
+    this.plugin = uppy.getPlugin(options.id)
   }
 
   componentWillUnmount () {

src/react/StatusBar.js

@@ -22,7 +22,7 @@ class StatusBar extends React.Component {
 
     uppy.use(StatusBarPlugin, options)
 
-    this.plugin = uppy.getPlugin('react:StatusBar')
+    this.plugin = uppy.getPlugin(options.id)
   }
 
   componentWillUnmount () {