Added check to verify that the attribute exists in provided entityId. (…

…#10)

* Added check to verify that the attribute exists in provided entityId.
Added error detail to the innerHTML to notify user of the problem with attribute provided.
This previously would cause an error and would crash lovelace cards that this custom element was used in.

* Updated README.md and VERSION.

* Update README.md

VERSION

@@ -1 +1 @@
-0.2.0
+0.3.0

state-attribute-element.js

@@ -11,8 +11,13 @@ class StateAttributeElement extends HTMLElement {
       this.state = hass.states[entityId].attributes[attr][sub_attribute]
     }
     const card = document.createElement('state-attribute-element');
-    if (this.state.length != 0 || show_empty === true) {
-      this.innerHTML = `${prefix_string}${this.state}${suffix_string}`
+    if (this.state) {
+      if (this.state.length != 0 || show_empty === true) {
+        this.innerHTML = `${prefix_string}${this.state}${suffix_string}`
+      }
+    }
+    else {
+      this.innerHTML = 'Attribute: ' + attr + ' does not exist in entity: ' + entityId + '.'
     }
   }
   setConfig(config) {