thermal: initialize data_frame in constructor
diff --git a/cr2/thermal.py b/cr2/thermal.py
index e3e5357..185c032 100644
--- a/cr2/thermal.py
+++ b/cr2/thermal.py
@@ -60,6 +60,7 @@
 
         self.basepath = basepath
         self.data_csv = ""
+        self.data_frame = pd.DataFrame()
         self.unique_word = unique_word
 
         if not os.path.isfile(os.path.join(basepath, "trace.txt")):