The remote server returned an error: (400) Bad Request while trying to Populate Solr Managed Schema

Error:
Job started: Index_Update_IndexName=sitecore_master_index|#Exception: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> SolrNet.Exceptions.SolrConnectionException: <?xml version="1.0" encoding="UTF-8"?>
<response>

<lst name="responseHeader">
  <int name="status">400</int>
  <int name="QTime">1</int>
</lst>
<lst name="error">
  <lst name="metadata">
    <str name="error-class">org.apache.solr.common.SolrException</str>
    <str name="root-error-class">org.apache.solr.common.SolrException</str>
  </lst>
  <str name="msg">undefined field _indexname</str>
  <int name="code">400</int>
</lst>
</response>
 ---> System.Net.WebException: The remote server returned an error: (400) Bad Request.
   at System.Net.HttpWebRequest.GetResponse()
   at HttpWebAdapters.Adapters.HttpWebRequestAdapter.GetResponse()
   at SolrNet.Impl.SolrConnection.GetResponse(IHttpWebRequest request)
   at SolrNet.Impl.SolrConnection.PostStream(String relativeUrl, String contentType, Stream content, IEnumerable`1 parameters)
   --- End of inner exception stack trace ---
   at SolrNet.Impl.SolrConnection.PostStream(String relativeUrl, String contentType, Stream content, IEnumerable`1 parameters)
   at SolrNet.Impl.SolrConnection.Post(String relativeUrl, String s)
   at SolrNet.Impl.LowLevelSolrServer.SendAndParseHeader(ISolrCommand cmd)
   at Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.PerformReset(ISolrOperations`1 operations, String indexName)
   at Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.PerformRebuild(Boolean resetIndex, Boolean optimizeOnComplete, IndexingOptions indexingOptions, CancellationToken cancellationToken)
   at Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.Rebuild(Boolean resetIndex, Boolean optimizeOnComplete)
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Sitecore.Reflection.ReflectionUtil.InvokeMethod(MethodInfo method, Object[] parameters, Object obj)
   at Sitecore.Jobs.JobRunner.RunMethod(JobArgs args)
   at (Object , Object )
   at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
   at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain, Boolean failIfNotExists)
   at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain)
   at Sitecore.Jobs.DefaultJob.DoExecute()
   at Sitecore.Abstractions.BaseJob.ThreadEntry(Object state)

Solution: Populate Solr Managed Schema for the concerned DB index, in this case, Master index. If you get a 503 populating the Solr Schema, stop Solr service, reset IIS and try populating again. If you have selected multiple DB indexes to populate Schema, just select the concerned DB index... in my case, i selected just the master index to populate instead of all...