Fixed bug that appears if you try to use read in weights for dimensionality reduction

src/analysis/ReadAnalysisFrames.cpp

@@ -81,7 +81,8 @@ ReadAnalysisFrames::ReadAnalysisFrames( const ActionOptions& ao ):
   if( wwstr.size()>0 ) {
     log.printf("\n");
     wham_pointer = dynamic_cast<bias::ReweightBase*>( weight_vals[0]->getPntrToAction() );
-    if( !wham_pointer->buildsWeightStore() ) wham_pointer = NULL;
+    if( !wham_pointer ) wham_pointer = NULL;
+    else if( !wham_pointer->buildsWeightStore() ) wham_pointer = NULL;
     if( wham_pointer && weight_vals.size()!=1 ) error("can only extract weights from one wham object");
   } else log.printf("  weights are all equal to one\n");
   requestArguments( arg );