Do not crash for unknown grammar/let fallback apply

Fixes #654

org.eclipse.tm4e.registry/src/main/java/org/eclipse/tm4e/registry/internal/AbstractGrammarRegistryManager.java

@@ -28,6 +28,7 @@
 
 import org.eclipse.core.runtime.content.IContentType;
 import org.eclipse.jdt.annotation.Nullable;
+import org.eclipse.tm4e.core.TMException;
 import org.eclipse.tm4e.core.grammar.IGrammar;
 import org.eclipse.tm4e.core.registry.IGrammarSource;
 import org.eclipse.tm4e.core.registry.IRegistryOptions;
@@ -164,16 +165,24 @@ protected AbstractGrammarRegistryManager(final IRegistryOptions options) {
 					continue;
 				}
 
+				try {
 				// look for a grammar provided by the same plugin as the content-type
-				var grammar = getGrammarForScope(binding.scope.getQualifiedName());
-				if (grammar != null) {
-					return grammar;
+					var grammar = getGrammarForScope(binding.scope.getQualifiedName());
+					if (grammar != null) {
+						return grammar;
+					}
+				} catch (TMException ex) {
+					// most likely not grammar found, continue
 				}
 
+				try {
 				// look for a grammar provided by any plugin
-				grammar = getGrammarForScope(binding.scope.getName());
-				if (grammar != null) {
-					return grammar;
+					var grammar = getGrammarForScope(binding.scope.getName());
+					if (grammar != null) {
+						return grammar;
+					}
+				} catch (TMException ex) {
+					// most likely not grammar found, continue
 				}
 			}
 		}